Add interface translation support

[and4po]

Hello!
We believe that this amazing translation platform should have multiple interface localizations.
As it is an Open Source Project, we think that the translation should be community-driven. Maybe hosted on your platform with a team and one project for each language.

PS: We are a Translators Community, and we would like to translate your service interface into Catalan.

[leonardotaddeitranslated]

Hi Ander, thanks for your suggestion!
We will take it into account and let you know in case we decide to go ahead with making Matecat's interface localizable.
I'll let you know as soon as we have any updates!

[and4po]

Great! Thanks!