I'm the only non-japanese in both of the dojos I attend, and am also the only one with English on my name bag. As everyone else's names are in kanji it can be a bit of a problem sometimes.... because I can't read kanji. Sometimes the kanji are easy to identify, such as Yamazaki or Otake, but most of the time they are extremely complex and written in a curved script which adds to the problem.
What I tend to do is remember things about the people instead. Are they tall, or fat, or do they have easily identifiable bogu and so on. In this way I have come to see all my teachers by how they look and act, and not by what they say (language barrier, remember?).
Hirai sensei - easy, he's always at the far right of the dojo because he's the highest ranked teacher (7th dan hanshi). He's a really nice old man who always smiles. I can always identify him even when he has his back turned by the way he holds himself, and by the way he doesn't move much. He generally beats the crap out of most people, but you can tell his age is getting to him :(
Grumpy sensei - he's next from the right, 7th dan koshi. I can never remember his name, but I'm trying. He is the one who always criticizes me: you're doing it wrong, no, like this, stop, let me show you, no, no, no (thus his name, which is meant affectionately). He also has a strange tendency to lean to one side, I think it's because his hip hurts.
Yamazaki sensei - has bright grey hair and dazzling white teeth. A little bit taller than the average Japanese person. He also has a yellow do, so I can find him easily (plus 'yama' is an easy kanji to spot). He's very aggressive, always gnashing his teeth and yelling. He's also very nice, he even invited me to go drinking haha.
Book sensei - I have no idea what his name is, but he gave me an English/Japanese kendo dictionary and ever since then I've thought of him as book-sensei. He's really cool, and I can spot him because he's a bit debu.
Sensei sensei - He's my teacher at the Sunday dojo. Quite tall and skinny, wears very old bogu and glasses. He is very nice and polite and smiles a lot, and always tries to teach me, thus his name.
And so on, will add more soon.