PageDataCollection languagesForSomePage = EPiServer.DataFactory.Instance.GetLanguageBranches(somePageLink);

I usually do it like that. Then check pagedata.LanguageBranch for each item in the collection.

Thanks Johan,
Your code work fine, now I get the supported language codes, like "en", "sv" etc. But in episerver edit mode, at the "Show page tree:" dropdown list I can see the full name of the language, like "English", "Svenska" etc. Ofcourse I can do an own translator of the codes but I prefer to use the same translator as EPiServer use for this. Do you know how to do this, I mean...how do I get "English" instead of "en"?

You should have that as a property on LanguageBranch:

LanguageBranch branch = LanguageBranch.Load(pagedata.LanguageID);

Yiihaa, Thanks alot Johan, that solves my problem.