Tesco didn't even get the ENGLISH right for that one (Lice or less), so what chance did they have with Welsh?

It should be 10 items or FEWER.

Rule:
LESS is singular, like LESS WATER
FEWER is plural, like FEWER PEOPLE or ITEMS

The opposite, MORE, is both singular and plural which may explain the confusion.

And I find a good rule with language translation tools like BABELFISH is to translate the output back into the original language. It's not often right!