I’m running a V4 forum on a (customized) translated language.

I assume most (?) of the language keys/phrases should stay pretty much the same, so I was wondering - will the upgrade script keep my existing translation, at least for the identical key phrases? or will I have to translate everything from scratch?

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Pinging @Esther E but I'm pretty sure the answer is yes. The vast majority of lang strings overlap with v4, and the way the language/translation system works hasn't changed much so I'd assume it should just use your existing translations.

Yep, although v5 is a big upgrade, it’s still just an upgrade so all translated strings remain translated.