Just a little trick I have found that works really well. You can guess their nationality initially, I am good at calibrating this as I have been travelling for two years, I can pick out regional variations in accents in Europe and even the USA. I can tell which countries Asians are from, like S. Korea, Japan, Viet Nam etc...
Just asking foreigners where they are from is an easy way to open and also you can use this fact for rapport assumption... later. You come across as being genuinely interested, as long as you don't come across as a salesman ie. a bad approach.

After the initial approach, instead of using an observation to break the ice, I used some simple questions but then deliberately mixed up the spoken language. I did this by accident one time and I have used it exclusively since.
The first question for example when you discover they are German: Wo Wohnst du im Deutschland? - Where do you live in Germany
The second question would be like: was ist deine lieblings fenster?
Literal translation: What is your favourite window?

This non sensical question will have give the following results. Either they will look at you bemused, repeat what you have said in your language or laugh hysterically. You then tell them that you were doing the fun test on them and then you can either say well done you passed, you might be worth getting to know better if they laugh. If they don't get it explain you were testing them and then they will usually start laughing and then tell them they nearly failed the fun test and then get them to qualify themselves some more eg. tell me two reasons why I would want to hang around with you.
If you get no reaction then eject. Seriously, don't waste your time talking to them, this is a good way of disqualifying miserable/ bad day people without any effort.

Couple of caveats: The first question should be said enthusiastically eg. oh really your from Germany, wo whonst...
Then the second should be done with a VERY serious facial expression, was ist deine....
Then you can substitute your own random shit in there. It works because it makes out like your really trying to converse with them in their language and they find it funny when you slip up! If they don't, then they have no sense of humour, having a bad day etc...

Whilst this isn't some amazing routine, it does break the ice well, disqualifies boring chicks, sets you up in a qualifying frame and as you are dealing with a foreign chick, you can go on to some easy rapport assumption. You can guarantee 99% of the time that a foreign chick will be a tourist of some kind and even if not, they have gone and done some touristy things at least. So you can start asking where they have been and make statements or observations on where they have been. This can be done using how you felt at these places. Think Gunwitch and the pop culture talk. You can literally have felt the same things and had the same experiences and this helps with that assumption.

So what applications does it have for you guys? Well if you know certain places where foreign chicks are going to be you can target your approach to their language eg. backpacker nights especially in London and other cities. Personally, I speak German well and have not tried to do it in any other language but with this alone I had Belgium, Holland, Sweeden, Denmark, Finland, Norway, Austia and Switzerland covered... most of Europe can speak German as these countries languages either incorporate it or are very, very similar. The same rules apply and of the field tests I have done it has opened most of the time and led on to a good rapport assumed.

If you want to open a large group of chicks, who are speaking the same language eg. groups of back packers, or whatever foreign group. You can go in with the same opener but as they are in a group you will probably get the bitch shield. Just plow through and say theres no need to be like that I'm only trying to learn a bit of your language. They will often ST you and say what do you want to know and then you have to reel a bit of fluff off about what you can speak. Its important to remain calm and unaffected. It does help if you can speak a bit of their language but its not essential to be really good, just a few sentences is adequate. More often than not, the AFOG of the group will isolate with you, to teach you and most often that means you are accepted in to the group and you can look for IOI's from the other chicks or sarge her.