Hi @initvector & @charrondev,
The OS forum is displayed in at least three languages here. German, English and French. See the screenshot below. You might want to check this out.
That's a known issue of the multi lingual plugin. The problem behind that is that the entry in the Activity table is an alread translated string and therefore cannot be translated again. This comment explains it very well: https://github.com/vanilla/addons/issues/155#issuecomment-251788000