I live in a foreign country and find I can still be successful with girls even though 95% of them don't speak my language. How? Just talk to them anyway in your own language.

By doing this you put them on your turf and in a position of needing to appease you (by first understanding you and second responding in your language). Don't overdo it to the point of frustrating them! Use a few simple phrases that people who don't speak your language might still understand - "How are you? What is your name?"

On the contrary, trying to speak in someone else's language puts you in their territory. Even if I do speak the local language, I only transition after I have gotten their full attention and basic communication/logistics becomes more important. They are often pleasantly surprised and relieved to be back in their own territory.

On a related note, If I am trying to show dominance in a situation (often times not related to "picking up girls"), I still talk talk talk in my language, even though it is clear that the other person doesn't understand. It is confusing to them and helps you maintain your own identity/stay in your own comfort zone (where you are strongest).