Merge translations for Catalan language

[jmaspons]

There are two codes for Catalan language in Transifex: ca and ca_ES. This is confusing and unnecessary. I would merge the translations and remove ca_ES as Catalan is also spoken in France, Andorra, and Italy. If you need help with conflicts, I can resolve them from the .ts files and send PRs.

Also, is it possible to work directly with the .ts files and send PRs instead of doing the translations in transifex?

[m-kuhn]

@beanzmo