I've been wondering, would the usage of translations of the game that weren't included in the game itself count as modding the game? For example, the copies of GTA:SA sold in Poland contained 2 discs, one with the English version of the game and the second with an installer of a Polish translation made by the publisher (Cenega in this case).

Only official version of the game should be used. If your polish patch was published by an official and licensed distributor of the game then it is allowed Pirate/fan made translation aren't allowed

LUL