[Hatix]

In japan, they know foreign people will have problems so they do things to make it easier for them. For example, in a macdonalds, there's a big paper on the counter with pictures of what they have. So if the one behind the counter sees any language difficulty they use simple english and point on the paper
Oh, and btw, they don't have mayonaise in macdonalds in Japan :p (I like mayo...)
Same goes for restaurants, with the plastic models (but you probably heard of those already).
And other shops, well, they always have a digital cash register or a calculator at hand with a digital display, so if you don't understand the amount they are saying, you can still read it (and they know that, so they are very helpful).
And be prepared to hear a LOT of "Irasshaimase!" when you enter a shop. (from everyone that works there, if you pass them).

So that's for the language barrier They do of course like it when you try to speak japanese
and like eddie said, bow or nod (as they will do it too)