Hey there @Mosh Ontong 👋
To answer:
But what I want is to select only the language's id
It seems like a valid concerned, and I opened this to internal discussion. As of Appwrite 1.3.1, inside Query.select, you should only specify your custom attributes. Internal attribudes such as ID, collectionId, databaseId, permissions, createdAt and updatedAt will always be returned.
We noticed this can be annoying and defeat purpose of select query. We initially let those internal attributes there to make offline support possible in future. Thanks to feedback such as yours, we can think about it a bit more to provide better developer experience :appwritepeepo:
Regarding normal select and select with relationships, they should work exactly the same. And for selecting data inside relation, you use . in between names. Sooo in your scenario doing Query.select(['language.$id']) would be the right approach. Again, sadly, this wont work as of right now. Buuut your thread started internal discussion 😇