Maybe it depence on the version of the host?

I'm figuring probably that, but shouldn't we be able to change it to our own ?

I tried a few things but I banged my head into the computer screen after I couldn't figure out how to...

someone told by it is ddepending on the steam civ5 's language setting ..; i found once , but i had no other choice than french alas ... maybe if i could have changed for english it 'd have worked ...

I have mine set to english as well.. i sometimes get it in many different languages.. it does appear to be whomever's turn is before mine.. which I think just doesn't make sense.

i meant it is depending of the settings of whom created (or maybe host) the game
....

I know i solve no questions but maybe it can help about this clue ...

From my experience it depends on the language setting of the player "causing" the message. For proposals for example, when erekose sends me a proposal that message is in French, while with most of the others players it is in English. If I send you a proposal you probably get the message in German. For city state messages I am not 100%sure, I think it depends on the player who has the turn before you.
Probably this is because of the hotseat mode that GMR is using

I think so as well. My Theory is that it depends on the player in front of you. I you end you turn the games starts calculating the turn of the next player before you can save the game. So when the next player loads the game this mesages are already generated by the game in the language the previous player use.

Proof: its probably not exactly proof but i had a least one game where the player in front of me quit and the language I received the messages in changed after that. Sometimes its even in languages the syste can't display so I obly get a practcally empty field.

It could also be that the messages from the CS are in the language of the player whose computer plays also for the CS and the barbarians. So it would be either the first human player or the last one in the current turn,

I feel like it is the language of the player of the previous turn. Messages that spawn at end of my previous turn (from city states, etc. are in my language, but anything at beginning of next turn will be what I am assuming is the previous players language, although I suppose it could be the host, but I think it has happened in games where I am host, so back to previous player theory.

I've had this happen in vanilla multiplayer so not sure it's a GMR thing.