Being too forceful with something as delicate as 
cels is rather foolish (and being too forceful 
with something that isn't yours is always rude) 
but I don't see what's wrong with helping yourself 
in a store instead of pestering the salesperson. 
Of course, if said shopper is rooting behind the 
counter and other places where they shouldn't be 
that's incredibly rude. But it seems to me if one 
is just browsing instead of looking for something 
in particular then bringing in the dealer would be 
counterproductive. Especially if the dealer was 
already engaged in a conversation with another 
customer!

If the shopper is not fluent in the native 
language, then I can see an even greater added 
incentive for not engaging a saleperson. Some 
people believe that it's always polite to try to 
communicate, but other people believe it would be 
polite not to annoy the salepeople with their 
ineptness in the language if at all possible. I've 
come across enough offduty salepeople complaining 
about how they're always being asked to do simple 
things that the customer should be able to 
accomplish themselves that I always try to be as 
self-sufficient as possible unless directly 
approached. (And since my attention is sometimes 
hard to get-- half the time I don't even respond 
to my own name... it just doesn't click in that 
"hey, that person is trying to talk to *me*!"-- I 
usually give the benefit of the doubt whether 
someone could be distracted/hard of hearing etc.)

I really don't see how the customer's grasp of the 
language nor their motives for traveling has 
anything to do with etiquette. It's always wise to 
learn the rudiments of a language when one travels 
afar, but sometimes that's not always possible. I 
know if I ever go to Japan the most comprehensible 
thing I'll be able to say is "Do you want to be 
the seme or the uke?" ;p~ I know Spanish, French, 
and Filippino (not to mention English, although 
after my current brain meltdown this semester, 
the only thing I fluently speak now is gibberish) 
but when I tried to learn Japanese after getting 
interested in anime, my head nearly imploded. 
(Partly because my brain is just full after so 
many years of schooling and partly because 
Japanese seems 10x as hard as S/F/F ^^;;;)

I also know of people who fly to London to go 
shopping, or fly to the USA only to see Disney 
World or catch a few matinees on Broadway or go 
shopping (for jeans... really.) If they're happy 
with that (and their happiness isn't hampering 
anyone else's happiness) then I'm happy for them.