gatsby-node.js

const path = require('path')
const language_config = require(`./src/components/localization/language-config.js`)

const translations_cache = {}
// Based upon https://github.com/gatsbyjs/gatsby/tree/master/examples/using-i18n
exports.onCreatePage = ({ page, actions }) => {
    const { createPage, deletePage } = actions

    // First delete the incoming page that was automatically created by Gatsby
    // So everything in src/pages/
    deletePage(page)
    Object.keys(language_config).map(lang => {
        // Use the values defined in "locales" to construct the path
        const { path, is_default } = language_config[lang]
        const localized_path = is_default ? page.path : `${path}${page.path}`

        if (!translations_cache[lang]) {
            const translation_json = require(`./src/translations/${lang}`)
            translations_cache[lang] = translation_json
        }

        return createPage({
            // Pass on everything from the original page
            ...page,
            // Remove trailing slash from page.path (e.g. "/de/")
            path: localized_path,
            // Pass in the locale as context to every page
            context: {
                ...page.context,
                locale: lang,
                localeResources: translations_cache[lang],
                pathname: localized_path,
            },
        })
    })
}

exports.onCreateWebpackConfig = ({ actions, getConfig }) => {
    const config = getConfig()
    if (config.optimization) config.optimization.minimizer[0].options.parallel = 2
    actions.setWebpackConfig({
        resolve: {
            modules: [path.resolve(__dirname, 'src'), 'node_modules'],
        },
    })
}