I think, when you override theme translations as an administrator, it only overrides the translations for whatever language you are using when you make the edits. The overrides don’t carry across to other languages. Does that line up with what you’re seeing?
The vast majority of forums use the default config, which doesn’t allow users to change their language, so this issue isn’t noticed.
It is a limitation we’re aware of, but we don’t have an ETA on improving it at the moment. Ideally, we’ll make it work the same as /admin/customize/text, so you can set overrides for each language. Certainly pr-welcome