I just did go in a game to make a quick test with English players (as I am a French one). I used the spray "Merci" (which means "Thanks"). While it appeared as "Merci" for me and another French player, it appeared as "Thanks" for the English ones.
So the sprays are definitely translated from one language to another.
Now, it doesn't apply to all the sprays, because some characters have sprays linked to their own background (their country), and so the language on those sprays is kept whatever which country you are coming from. But you can recognize those easily, as they are not translated in your own language either (except if the character is from your own country of course).