diff --git a/tests/deepLTranslate.test.ts b/tests/deepLTranslate.test.ts
new file mode 100644
index 0000000..1d66ec7
--- /dev/null
+++ b/tests/deepLTranslate.test.ts
@@ -0,0 +1,86 @@
+import { deepLTranslate } from '../src/sheetsl';
+
+describe('deepLTranslate', () => {
+  beforeEach(() => {
+    global.PropertiesService = {
+      getUserProperties: jest.fn(() => ({
+        getProperty: jest.fn(() => 'SampleApiKey:fx'),
+      })),
+    } as unknown as GoogleAppsScript.Properties.PropertiesService;
+    global.encodeURIComponent = jest.fn(
+      (text: string) => text,
+    ) as unknown as typeof encodeURIComponent;
+  });
+  afterEach(() => {
+    jest.clearAllMocks();
+  });
+  const mockSourceObjects = [
+    {
+      note: 'with source language specified',
+      sourceLang: 'EN-US',
+      targetLang: 'DE',
+      sourceText: 'Hello, World!',
+      translatedText: 'Hallo, Welt!',
+    },
+    {
+      note: 'without source language specified',
+      sourceLang: null,
+      targetLang: 'DE',
+      sourceText: 'Hello, World!',
+      translatedText: 'Hallo, Welt!',
+    },
+    {
+      note: 'in an array of strings',
+      sourceLang: null,
+      targetLang: 'DE',
+      sourceText: ['Hello, World!', 'Hello, World!'],
+      translatedText: ['Hallo, Welt!', 'Hallo, Welt!'],
+    },
+  ];
+  it.each(mockSourceObjects)(
+    'should translate text $note',
+    // eslint-disable-next-line @typescript-eslint/no-unused-vars
+    ({ note, sourceLang, targetLang, sourceText, translatedText }) => {
+      global.UrlFetchApp = {
+        fetch: jest.fn(() => ({
+          getContentText: jest.fn(() =>
+            JSON.stringify({
+              translations: Array.isArray(translatedText)
+                ? translatedText.map((text) => ({ text: text }))
+                : [{ text: translatedText }],
+            }),
+          ),
+          getResponseCode: jest.fn(() => 200),
+        })),
+      } as unknown as GoogleAppsScript.URL_Fetch.UrlFetchApp;
+      const translated = deepLTranslate(sourceText, sourceLang, targetLang);
+      expect(translated).toStrictEqual(
+        Array.isArray(translatedText) ? translatedText : [translatedText],
+      );
+      expect(global.UrlFetchApp.fetch).toHaveBeenCalled();
+    },
+  );
+  const mockSourceObjectsError = [
+    {
+      note: 'when source text is null',
+      sourceLang: 'EN-US',
+      targetLang: 'DE',
+      sourceText: null,
+    },
+    {
+      note: 'when source text is empty',
+      sourceLang: 'EN-US',
+      targetLang: 'DE',
+      sourceText: '',
+    },
+  ];
+  it.each(mockSourceObjectsError)(
+    'should throw an error $note',
+    // eslint-disable-next-line @typescript-eslint/no-unused-vars
+    ({ note, sourceLang, targetLang, sourceText }) => {
+      expect(() => deepLTranslate(sourceText, sourceLang, targetLang)).toThrow(
+        new Error('[SheetsL] Empty input.'),
+      );
+    },
+  );
+});
diff --git a/tests/getDeepLApiBaseUrl.test.ts b/tests/getDeepLApiBaseUrl.test.ts
index 43d7bb1..0010382 100644
--- a/tests/getDeepLApiBaseUrl.test.ts
+++ b/tests/getDeepLApiBaseUrl.test.ts
@@ -1,19 +1,15 @@
 import { getDeepLApiBaseUrl } from '../src/sheetsl';
 
-const DEEPL_API_VERSION = 'v2';
-const DEEPL_API_BASE_URL_FREE = `https://api-free.deepl.com/${DEEPL_API_VERSION}/`;
-const DEEPL_API_BASE_URL_PRO = `https://api.deepl.com/${DEEPL_API_VERSION}/`;
-
 const patterns = [
   {
     title: 'DeepL API Free account',
     input: 'xxxxxxxxxxx:fx',
-    expectedOutput: DEEPL_API_BASE_URL_FREE,
+    expectedOutput: 'https://api-free.deepl.com/v2/',
   },
   {
     title: 'DeepL API Pro account',
     input: 'xxxxxxxxxxx',
-    expectedOutput: DEEPL_API_BASE_URL_PRO,
+    expectedOutput: 'https://api.deepl.com/v2/',
   },
 ];
 
diff --git a/tests/setLanguage.test.ts b/tests/setLanguage.test.ts
new file mode 100644
index 0000000..25155a7
--- /dev/null
+++ b/tests/setLanguage.test.ts
@@ -0,0 +1,320 @@
+import { setLanguage } from '../src/sheetsl';
+
+describe('setLanguage', () => {
+  beforeEach(() => {
+    // eslint-disable-next-line @typescript-eslint/no-empty-function
+    jest.spyOn(console, 'error').mockImplementation(() => {});
+  });
+  afterEach(() => {
+    jest.clearAllMocks();
+  });
+  it('should set the language', () => {
+    global.SpreadsheetApp = {
+      getUi: jest.fn(() => ({
+        Button: { OK: 'ok' },
+        ButtonSet: { OK_CANCEL: 'ok_cancel' },
+        prompt: jest
+          .fn()
+          .mockReturnValueOnce({
+            // prompt for source language
+            getSelectedButton: jest.fn(() => 'ok'),
+            getResponseText: jest.fn(() => 'EN'),
+          })
+          .mockReturnValueOnce({
+            // prompt for target language
+            getSelectedButton: jest.fn(() => 'ok'),
+            getResponseText: jest.fn(() => 'DE'),
+          }) as unknown as GoogleAppsScript.Base.PromptResponse,
+        alert: jest.fn(),
+      })),
+    } as unknown as GoogleAppsScript.Spreadsheet.SpreadsheetApp;
+    global.PropertiesService = {
+      getUserProperties: jest.fn(() => ({
+        getProperties: jest.fn(() => ({})), // No existing settings
+        getProperty: jest.fn(() => 'Sample-API-Key:fx'),
+        setProperties: jest.fn(),
+      })),
+    } as unknown as GoogleAppsScript.Properties.PropertiesService;
+    global.UrlFetchApp = {
+      fetch: jest
+        .fn()
+        .mockReturnValueOnce({
+          // deepLGetLanguages('source')
+          getContentText: jest.fn(() =>
+            JSON.stringify([
+              { language: 'EN', name: 'English' },
+              { language: 'DE', name: 'German' },
+            ]),
+          ),
+          getResponseCode: jest.fn(() => 200),
+        })
+        .mockReturnValueOnce({
+          // deepLGetLanguages('target')
+          getContentText: jest.fn(() =>
+            JSON.stringify([
+              { language: 'EN-US', name: 'English (US)' },
+              { language: 'DE', name: 'German' },
+            ]),
+          ),
+          getResponseCode: jest.fn(() => 200),
+        }),
+    } as unknown as GoogleAppsScript.URL_Fetch.UrlFetchApp;
+    setLanguage();
+    expect(global.UrlFetchApp.fetch).toHaveBeenCalledTimes(2);
+    expect(console.error).not.toHaveBeenCalled();
+  });
+  describe('should catch an error', () => {
+    afterEach(() => {
+      jest.clearAllMocks();
+    });
+    it('when the user cancels the language setup after notification of existing settings', () => {
+      global.SpreadsheetApp = {
+        getUi: jest.fn(() => ({
+          Button: { YES: 'yes' },
+          ButtonSet: { YES_NO: 'yes_no' },
+          prompt: jest.fn(),
+          alert: jest.fn(() => 'no'), // cancel the setup
+        })),
+      } as unknown as GoogleAppsScript.Spreadsheet.SpreadsheetApp;
+      global.PropertiesService = {
+        getUserProperties: jest.fn(() => ({
+          getProperties: jest.fn(() => ({
+            targetLocale: 'DE', // existing settings
+          })),
+        })),
+      } as unknown as GoogleAppsScript.Properties.PropertiesService;
+      global.UrlFetchApp = {
+        fetch: jest.fn(),
+      } as unknown as GoogleAppsScript.URL_Fetch.UrlFetchApp;
+      setLanguage();
+      expect(global.UrlFetchApp.fetch).not.toHaveBeenCalled();
+      expect(console.error).toHaveBeenCalledWith(
+        expect.stringMatching(
+          /^Error: \[SheetsL\] Canceled: Language setting has been canceled.\n/,
+        ),
+      );
+    });
+    it('when the user cancels the language setup after prompted to enter source locale', () => {
+      global.SpreadsheetApp = {
+        getUi: jest.fn(() => ({
+          Button: { OK: 'ok' },
+          ButtonSet: { OK_CANCEL: 'ok_cancel' },
+          prompt: jest.fn().mockReturnValueOnce({
+            // prompt for source language
+            getSelectedButton: jest.fn(() => 'cancel'), // cancel the setup after prompted to enter source locale
+            getResponseText: jest.fn(() => 'EN'),
+          }) as unknown as GoogleAppsScript.Base.PromptResponse,
+          alert: jest.fn(),
+        })),
+      } as unknown as GoogleAppsScript.Spreadsheet.SpreadsheetApp;
+      global.PropertiesService = {
+        getUserProperties: jest.fn(() => ({
+          getProperties: jest.fn(() => ({})), // No existing settings
+          getProperty: jest.fn(() => 'Sample-API-Key:fx'),
+          setProperties: jest.fn(),
+        })),
+      } as unknown as GoogleAppsScript.Properties.PropertiesService;
+      global.UrlFetchApp = {
+        fetch: jest
+          .fn()
+          .mockReturnValueOnce({
+            // deepLGetLanguages('source')
+            getContentText: jest.fn(() =>
+              JSON.stringify([
+                { language: 'EN', name: 'English' },
+                { language: 'DE', name: 'German' },
+              ]),
+            ),
+            getResponseCode: jest.fn(() => 200),
+          })
+          .mockReturnValueOnce({
+            // deepLGetLanguages('target')
+            getContentText: jest.fn(() =>
+              JSON.stringify([
+                { language: 'EN-US', name: 'English (US)' },
+                { language: 'DE', name: 'German' },
+              ]),
+            ),
+            getResponseCode: jest.fn(() => 200),
+          }),
+      } as unknown as GoogleAppsScript.URL_Fetch.UrlFetchApp;
+      setLanguage();
+      expect(global.UrlFetchApp.fetch).toHaveBeenCalledTimes(2);
+      expect(console.error).toHaveBeenCalledWith(
+        expect.stringMatching(
+          /^Error: \[SheetsL\] Canceled: Language setting has been canceled.\n/,
+        ),
+      );
+    });
+    it('when the user enters a value not included in the list of available source languages', () => {
+      global.SpreadsheetApp = {
+        getUi: jest.fn(() => ({
+          Button: { OK: 'ok' },
+          ButtonSet: { OK_CANCEL: 'ok_cancel' },
+          prompt: jest.fn().mockReturnValueOnce({
+            // prompt for source language
+            getSelectedButton: jest.fn(() => 'ok'),
+            getResponseText: jest.fn(() => 'JA'), // a source locale that is not included in the response value of deepLGetLanguages('source')
+          }) as unknown as GoogleAppsScript.Base.PromptResponse,
+          alert: jest.fn(),
+        })),
+      } as unknown as GoogleAppsScript.Spreadsheet.SpreadsheetApp;
+      global.PropertiesService = {
+        getUserProperties: jest.fn(() => ({
+          getProperties: jest.fn(() => ({})), // No existing settings
+          getProperty: jest.fn(() => 'Sample-API-Key:fx'),
+          setProperties: jest.fn(),
+        })),
+      } as unknown as GoogleAppsScript.Properties.PropertiesService;
+      global.UrlFetchApp = {
+        fetch: jest
+          .fn()
+          .mockReturnValueOnce({
+            // deepLGetLanguages('source')
+            getContentText: jest.fn(() =>
+              JSON.stringify([
+                { language: 'EN', name: 'English' },
+                { language: 'DE', name: 'German' },
+              ]),
+            ),
+            getResponseCode: jest.fn(() => 200),
+          })
+          .mockReturnValueOnce({
+            // deepLGetLanguages('target')
+            getContentText: jest.fn(() =>
+              JSON.stringify([
+                { language: 'EN-US', name: 'English (US)' },
+                { language: 'DE', name: 'German' },
+              ]),
+            ),
+            getResponseCode: jest.fn(() => 200),
+          }),
+      } as unknown as GoogleAppsScript.URL_Fetch.UrlFetchApp;
+      setLanguage();
+      expect(global.UrlFetchApp.fetch).toHaveBeenCalledTimes(2);
+      expect(console.error).toHaveBeenCalledWith(
+        expect.stringMatching(
+          /^Error: \[SheetsL\] Invalid Value \(JA\): Enter a valid value.\n/,
+        ),
+      );
+    });
+    it('when the user cancels the language setup after prompted to enter target locale', () => {
+      global.SpreadsheetApp = {
+        getUi: jest.fn(() => ({
+          Button: { OK: 'ok' },
+          ButtonSet: { OK_CANCEL: 'ok_cancel' },
+          prompt: jest
+            .fn()
+            .mockReturnValueOnce({
+              // prompt for source language
+              getSelectedButton: jest.fn(() => 'ok'),
+              getResponseText: jest.fn(() => 'EN'),
+            })
+            .mockReturnValueOnce({
+              // prompt for target language
+              getSelectedButton: jest.fn(() => 'cancel'), // cancel the setup after prompted to enter target locale
+              getResponseText: jest.fn(() => 'DE'),
+            }) as unknown as GoogleAppsScript.Base.PromptResponse,
+          alert: jest.fn(),
+        })),
+      } as unknown as GoogleAppsScript.Spreadsheet.SpreadsheetApp;
+      global.PropertiesService = {
+        getUserProperties: jest.fn(() => ({
+          getProperties: jest.fn(() => ({})), // No existing settings
+          getProperty: jest.fn(() => 'Sample-API-Key:fx'),
+          setProperties: jest.fn(),
+        })),
+      } as unknown as GoogleAppsScript.Properties.PropertiesService;
+      global.UrlFetchApp = {
+        fetch: jest
+          .fn()
+          .mockReturnValueOnce({
+            // deepLGetLanguages('source')
+            getContentText: jest.fn(() =>
+              JSON.stringify([
+                { language: 'EN', name: 'English' },
+                { language: 'DE', name: 'German' },
+              ]),
+            ),
+            getResponseCode: jest.fn(() => 200),
+          })
+          .mockReturnValueOnce({
+            // deepLGetLanguages('target')
+            getContentText: jest.fn(() =>
+              JSON.stringify([
+                { language: 'EN-US', name: 'English (US)' },
+                { language: 'DE', name: 'German' },
+              ]),
+            ),
+            getResponseCode: jest.fn(() => 200),
+          }),
+      } as unknown as GoogleAppsScript.URL_Fetch.UrlFetchApp;
+      setLanguage();
+      expect(global.UrlFetchApp.fetch).toHaveBeenCalledTimes(2);
+      expect(console.error).toHaveBeenCalledWith(
+        expect.stringMatching(
+          /^Error: \[SheetsL\] Canceled: Language setting has been canceled.\n/,
+        ),
+      );
+    });
+    it('when the user enters a value not included in the list of available source languages', () => {
+      global.SpreadsheetApp = {
+        getUi: jest.fn(() => ({
+          Button: { OK: 'ok' },
+          ButtonSet: { OK_CANCEL: 'ok_cancel' },
+          prompt: jest
+            .fn()
+            .mockReturnValueOnce({
+              // prompt for source language
+              getSelectedButton: jest.fn(() => 'ok'),
+              getResponseText: jest.fn(() => 'DE'),
+            })
+            .mockReturnValueOnce({
+              // prompt for target language
+              getSelectedButton: jest.fn(() => 'ok'),
+              getResponseText: jest.fn(() => 'JA'), // a target locale that is not included in the response value of deepLGetLanguages('target')
+            }) as unknown as GoogleAppsScript.Base.PromptResponse,
+          alert: jest.fn(),
+        })),
+      } as unknown as GoogleAppsScript.Spreadsheet.SpreadsheetApp;
+      global.PropertiesService = {
+        getUserProperties: jest.fn(() => ({
+          getProperties: jest.fn(() => ({})), // No existing settings
+          getProperty: jest.fn(() => 'Sample-API-Key:fx'),
+          setProperties: jest.fn(),
+        })),
+      } as unknown as GoogleAppsScript.Properties.PropertiesService;
+      global.UrlFetchApp = {
+        fetch: jest
+          .fn()
+          .mockReturnValueOnce({
+            // deepLGetLanguages('source')
+            getContentText: jest.fn(() =>
+              JSON.stringify([
+                { language: 'EN', name: 'English' },
+                { language: 'DE', name: 'German' },
+              ]),
+            ),
+            getResponseCode: jest.fn(() => 200),
+          })
+          .mockReturnValueOnce({
+            // deepLGetLanguages('target')
+            getContentText: jest.fn(() =>
+              JSON.stringify([
+                { language: 'EN-US', name: 'English (US)' },
+                { language: 'DE', name: 'German' },
+              ]),
+            ),
+            getResponseCode: jest.fn(() => 200),
+          }),
+      } as unknown as GoogleAppsScript.URL_Fetch.UrlFetchApp;
+      setLanguage();
+      expect(global.UrlFetchApp.fetch).toHaveBeenCalledTimes(2);
+      expect(console.error).toHaveBeenCalledWith(
+        expect.stringMatching(
+          /^Error: \[SheetsL\] Invalid Value \(JA\): Enter a valid value.\n/,
+        ),
+      );
+    });
+  });
+});