i have tried this code: 

var user = EPiServerProfile.Current;
user.Language = "da";
user.Save();

i changes the language in the database and in the "Personal Language" setting but episerver still uses the old language.

By the way if i change the language in episerver in the "Personal Language" setting then it works

Hi Lars

Try adding a call to ICurrentUiCulture.Save(string userName, CultureInfo culture) to your code-piece.

Hi Lars,

Try the below code-piece.

var profile = EPiServerProfile.Wrap(ProfileBase.Create(result.Username));
profile.Language = "da";
profile.Save();

adding this fixed the problem:

ServiceLocator.Current.GetInstance<ICurrentUiCulture>().Save(user.UserName, CultureInfo.GetCultureInfo("da"));

Thanks Stefan

Any idear where the information is saved in the database?