Could you include on the game pages that the developers can deposit which languages the game supports?

That would certainly often be a selling point. Currently I have to download the games first to see if it supports my desired language, because in the game description is almost never something about it. 

Yes, actually, there's a field for it in the More Information box, that can be set from the Metadata tab when editing the game. By all means, encourage game creators to use it.

Add that if the creator of the game has indicated the languages in metadata, these will be displayed under "more information" on the game page.

You can filter with "lang-" + language letter in the URL, for example:

All game with support Spanish
https://itch.io/games/lang-es

Or all game with Spanish latin america:
https://itch.io/games/lang-es-419