Hello. I had the same problem. My solution was:
1. Go to administration - site administration - language - language settings: from the drop down list of languages in "default language" select the language desired.
2. This will make that only NEW users will have by default, the language selected previously after they login. Old users will still have the english language when login, and will have to change manually in preferences menú. If you want, you could delete the user and créate the account again. This will be another solution.