Add Catalan translation

[corretge]

Added the /ca folder with all the items translated to Catalan.

Added to the README.md list of languages and sort alphabetically.

[yagil]

@corretge thank you for the awesome contribution!

[yagil]

@corretge would it be possible to revert the change to the README? Please of course add yourself next to Catalan! But the ordering of the languages should ideally remain in the order of addition to the repo.

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