I wanna know if there is a way to add languages to the games on the store, because I have the Italian retail version of TR4 but on GOG there isn't that localisation.

You can always contact GOG support https://support.gog.com/hc/en-us/requests/new?form=other&product=gog
and ask, whether they are interested in those files.

Just be aware: it is possible, that there exist different licences in regard to different languages for a game.
Therefore, if GOG only has the right to sell a game in a certain language (or certain languages), they may not be able to accept your files.

Edit: IF GOG had the right to sell the Italian version of TR4, they probably would sell that version.