Will the game be available in other languages? Stronghold is one of the few games where the translation from English and voice acting was great in French, so I'd really like to be able to get the game in that language

Doesn't seem so. Only in very rare cases GOG games feature multiple languages - which is quite a pity if you ask me, particularly in case of games where the English version cannot be considered the original one (such as it is with Stronghold).

Maybe the translation was from German actually, I didn't look into it. But yeah, I guess it's a pain to track down all the different distributors that have rights to the game in various countries, and who would each want their cut in the case of a multi-language release, I guess.

Still, it's a pity to let good localized versions die. I guess abandonware is gonna be the only way to get those titles in anything else than english for a long time.