Years ago I used to go into a Caffe Nero near Oxford Circus, where I worked. The baristas were all young, and either Italian or Polish. They did the usual fantastic job, and all with competence and good humour. One of them, an Italian girl, would always ask customers who didn't present a loyalty card to be stamped, "Do you have-a theese-a one?" and would point to the new cards.
I thought if it was quiet there one day I'd correct her discreetly, tell her what she was trying to say was, "Do you have one of these?" But she was always very kind to me, and to all the other harassed people in and out for their caffeine fix, and maybe saying that might have hurt her feelings.
And anyway, that's just never been my habit. London is full of people speaking the most appalling English - I include Londoners in that - and for my whole life I've always worked out what people have been trying to say, noted it, and answered accordingly.
Thirdly, it's kind of rude to correct people if you understand them. I had a rather annoying Polish friend for a while when I lived over in Warsaw, and he would say, "Please correct my English for me," and firstly I thought, 'Well, I'm not a free English lesson,' and secondly, if I'd done that he'd have barely been able to speak for more than a minute or two, as his English was fairly crap - it didn't matter: I could understand it, so there was no problem.
But I think the main reason I didn't correct the Caffe Nero barista was that I just enjoyed the way she said it: it was beautiful, and there is no reason why we should all speak any language in exactly the same way.