To be accurate it is not that you have to have content in Catalan (although that seems to be the _main_ purpose of the sponsored TLD) but you have to "be a part of the Catalan Linguistic and Cultural Community" as set forth in the registry agreement between ICANN and domini.cat [0]

[0] (go to specification 12) https://itp.cdn.icann.org/en/files/registry-agreements/cat/c...