Source language
The original language and locale that translators work from, such as en-US, before versions for other locales are created.
What it is
The source language is the language content is authored in before it is sent for translation. It should usually be a specific locale such as en-US, not the loose label 'English', because spelling, terminology, date examples, and even quoted interface text can differ by region: color or colour, cart or basket, 3/4/2026 meaning March or April. Every target translation points back to this source text.
Choose it when setting up the message catalog, TMS project, or XLIFF exchange. Keep source strings attached to stable message IDs and record changes, since an edit can invalidate existing translations and reopen work. Clear source copy is part of localization quality: ambiguity in the original multiplies across every target locale.
Gotcha: source language is not the same as fallback language. The app may fall back to English at runtime, but that is a display policy rather than translation provenance. Avoid translating through a second language when the original is available, since a pivot translation can carry its own lost nuance into every later target.
Ask AI for it
Set en-US as the source language for this localization project. Export the catalog as XLIFF 2.1 with srcLang='en-US', stable unit IDs, the source text, developer notes, and named placeholder metadata. Treat every other locale as a target and preserve a source revision for each unit so changed English reopens its translations for review. Do not use the runtime fallback locale as proof of source provenance, and do not translate a target locale from another target when the en-US source is available.