The Monero command line tools can be translated in various languages.
In order to use the same translation workflow as the Monero Core GUI, they use Qt Linguist translation files. However, to avoid the dependencies on Qt this normally implies, they use a custom loader to read those files at runtime.
In order to create, update or build translations files, you need to have Qt tools installed. For translating, you need either the Qt Linguist GUI (part of QT Creator or a 3rd-party standalone version), or another tool that supports Qt ts files, such as Transifex. The files are XML, so they can be edited in any plain text editor if needed.
You do not need anything from Qt in order to use the final translations.
To update ts files after changing source code:
./utils/translations/update-translations.sh
To add a new language, eg Spanish (ISO code es):
cp translations/monero.ts translations/monero_es.ts
To edit translations for Spanish:
linguist translations/monero_es.ts
To build translations after modifying them:
./utils/translations/build-translations.sh
To test a translation:
LANG=es ./build/release/bin/monero-wallet-cli
To add new translatable strings in the source code:
Use the tr(string) function if possible. If the code is in a class, and this class doesn't already have a tr() static function, add one, which uses a context named after what lupdate uses for the context, usually the fully qualified class name (eg, cryptonote::simple_wallet). If you need to use tr() in code that's not in a class, you can use the fully qualified version (eg, simple_wallet::tr) of the one matching the context you want. Use QT_TRANSLATE_NOOP(string) if you want to specify a context manually.
If you're getting messages of the form:
Class 'cryptonote::simple_wallet' lacks Q_OBJECT macro
all is fine, we don't actually need that here.