Hi teamanx and welcome to Tiki Community!
The best way is that you get commit access. It's very easy. See here: Commit
And the best would be that you commit your improvements to es/language.php in branches Tiki7 AND proposals/6x, since 6x is the new LTS (Long Term Support) version.
Ask again if in doubt (here or in the Spanish forum, of course ;-)
Xavi (from Barcelona)