Discourse sent only translation overrides for the current language to the client instead of sending overrides from fallback locales as well. This especially impacted en_GB -> en since most overrides would be done in English instead of English (UK).
This also adds lots of tests for previously untested code.
There's a small caveat: The client currently doesn't handle fallback locales for MessageFormat strings. That is why overrides for those strings always have a higher priority than regular translations. So, as an example, the lookup order for MessageFormat strings in German is:
1. override for de
2. override for en
3. value from de
4. value from en
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