Be yourself. If she likes you, she will choose you for you. And that is truly what you want. Other than that all you can do is actually ask her out.
I'd like to get a foreign (English speaking) girlfriend, but my friend who is native in English and I are trying to get the same girl. What should I do to get her despite of the fact that I'm having a disadvantage in language?
I can empathize with your dilema .. You do have a language barrier, yes .. But, romantic preference is based on many things .. Not just spoken language. Preference will come down to you individual personalities. Focus on impressing her with "who" you are as a person ... Not how you speak.
A romantic connection is made when two people actually 'click' (so to speak) on many levels. If, in the end, she prefers this other guy, it won't be just about what language he speaks .. on the otherhand, if she prefers your attentions, again, it won't be about what language you speak .. it will be about how you make her feel.
You should realize that you don't "get a girlfriend". You meet someone and get to know them and see if you both "click". Keep in mind too that if you narrow the field in the types of people you're interested in .. You could be missing the right person for you.