How do languages work in ressource packs ?

In order to make my datapacks more accessible, I'm thinking about intoducing a resource pack to manage different languages. I'm wondering what would happen if a player is using a language in which I did not provide a translator key. For example, let's say the player is using spanish and I want to translate example.string but did not implemented it in es_es.json. Will the game use a default language to translate it and if yes, what is this language ? Probably en_us but I don't know...