It seems to me that the issue here is that some users are (allegedly) discussing translations outside translatewiki.net. Can you ask the translator to get those users to discuss here?
I'm working here to translate MediaWiki messages because I'm administrator of Galician Wikipedia, I translated messages of OSM and other applications because I wanted to help that tools, but it isn't my goal. I know that at Galician Wikipedia, and the other MediaWiki sites, nobody discuss about translations, if somebody find an error, it is advised at Galician disscussion page, and then I fix it here. Other users of Galician Wikipedia make translations too, and we try to do the same kind of translation.
I don't mind about the translations of other tools, because I can't spend time with that work, but I need that MediaWiki translations don't be changed without a previous discussion and consensus at Galician Wikipedia because the changes affect a lot of users. If other communities wanted to make other kind of translations at their tools, greatǃ Bye.
Could it be just a mistake where he changes all translations in all projects without noticing? If so, maybe better directions on using Special:SearchTranslations may help. If a user is "write only" and doesn't see discussions, an option is a short block to attract attention, but I'd prefer to avoid it.
You can see his talk page, I tryed to talk with him several times to try reach an agreement, but it couldn't be possible. Finally I asked him to not modify MediaWiki messages without previous consensus at Galician Wikipedia, but last day he modified that translations.
He reads discussions, you can view his responses at talk page. I don't want a block for him, but I think that a message from other user telling him to have more care with his changes and to not modify MediaWiki translations without previous consensus, it could be useful... Bye.