Send is localized in over 50 languages. We use the fluent library and store our translations in FTL files in public/locales/. en-US is our base language, and other languages are managed by pontoon.
Strings are added or removed from [public/locales/en-US/send.ftl] as needed. Strings MUST NOT be changed after they've been commited and pushed to master. Changing a string requires creating a new ID with a new name (preferably descriptive instead of incremented) and deletion of the obsolete ID. It's often useful to add a comment above the string with info about how and where the string is used.
Once new strings are commited to master they are available for translators in Pontoon. All languages other than en-US should be edited via Pontoon. Translations get automatically commited to the github master branch.
The development environment includes all locales in public/locales via the L10N_DEV environment variable. Production uses package.json as the list of locales to use. Once a locale has enough string coverage it should be added to package.json.
In app/ we use the state.translate() function to translate strings to the best matching language base on the user's Accept-Language header. It's a wrapper around fluent's FluentBundle.format. It works the same for both server and client side rendering.
// simple string
const finishedString = state.translate('downloadFinish')
// with parameters
const progressString = state.translate('downloadingPageProgress', {
filename: state.fileInfo.name,
size: bytes(state.fileInfo.size)
})