Chat translations do work on the old chat!
Here is how to get it working: In-Experience Text Chat Translations - #98 by Tomi1231
The LegacyChat has the translation system built in, what is required to get it working is overwriting a variable set by a FFlag (which will probably be true soonish) and having a BoolValue named “ChatTranslationEnabled” set to true
I looked at the scripts from this custom chat module, and they seem to have all the translation stuff already in them (surprisingly, from the update I did in november). It might be outdated though, I didn’t check that. Might update the module just to be sure, the scripts concerned aren’t the same as the ones I have modified