Well... I am a Japanese national so maybe others won't have the same chance. I will share my experience:
I only used one of the 3 agencies, their office is just a small room in another building. They don't have rooms by themselves, so you will have a little chat with the front guy. You need to get past through that first.
I am a person who likes to be prepared and if I fail, at least I fail with decency. So I would suggest to go well dressed, maybe that can help.
Once I met the woman which was between 25 to 30. I quickly found out that she wasn't from Spain as the agency and their website advertised. We started to talk and play, but before any action, I told her that I have been abroad to many places and that I can speak four languages, once I finally convinced her, she opened up and told me personal stuff.
By the end of the session, she gave me her private phone number and asked me to become her boyfriend. Not really but more like saying: "-You can become my customer and contact me without going through the agency again-". To what gave me the impression that girls there may not mind at all. Since the girls are foreigners, I don't think they will discriminate other foreigners. Just be good at the front desk office...
...and don't do this:
http://forum.tokyoadultguide.com/esute-pink-salon-massage-reviews/5765-himawari-korean-sute-10.html