Add a transifex config and add the languages that have been translated

I tried to get the tx commands in as meson targets, but that doesn't worry because I can't make a target outside of the build directory.
Also the client seems to be broken in some fun ways, like I have to use an environment variable for the token and there no way to download translations without a local file first. This should all probably be documented at some point, but I'm hoping some of it will be resolved and we can avoid documenting it.