You may want to add a custom language that is derived from an existing language. It's useful if you want to change some translations for another instance of the application or for a specific customer.
The structure of a custom language is : existing language code + "-" + custom text.
Custom languages can be added like any other language and fallbacks work as expected.
It means that if the
en-microsoft.some_key is missing, then it will fallback to
en.some_key. So you only need to translate keys that should be customized.
Note that fallback are chained, so
fr-be-custom will fallback to
fr-be that will
itself fallback to
Using GetText syntax, it will only fallback to the source language. So either you create a fallback mechanism by yourself or you avoid fallbacking by translating everything in Translation.io for that custom language.