"Esgidiau rhedeg oren"
Translation:Orange running shoes
Something I've read in the past from a mod made it seem like American English was what the site is mainly geared towards. So while I can see why it might possibly be confusing, I don't think it would really disqualify "tennis shoes" from being a correct answer. Also, when I've seen other words (can't think of them off the top of my head) that mean something else in America vs Britain they just expect you to type what works for you.
It shouldn't be a major problem and they will probably acept both. While the site is generally geared towards American English I don't think it makes sense for the Welsh course to be. The differences between North and south Welsh seem to be more significant than between British and American English anyway so it isn't really worth worrying about.
The point is semantics. Trainers, sneakers, running shoes, tennis shoes... as long as they're sports shoes of some kind and you understand that "esgidiau rhedeg" translates as that then you'll be fine. You just need to remember that English has a far greater wealth of vocabulary compared to other languages and very often has a myriad of ways to express something. As long as you understand "esgidiau rhedeg" it doesn't really matter what other countries call it, only that you understand what it means to you.