mijn verjaarday is op zeven mei
I got the sentence "mijn verjaardag is op seven mei" to which I replied "my birthday is on the 7th of may' but it didn't accept 7th. I think the translation should accept the 7th because that is how Americans most commonly write out the name of the day.
Sure, but that way you're not learning/practicing how to say 9000 in Spanish. I think it makes sense to not accept numerals when translating to your target language (Spanish, in your case).
I don't see the harm of accepting numerals in English sentences, which is what the original post refers to.
Btw I think you mean zeven in the Dutch sentence in your actual post (not title) and verjaardag in your title :) :)
That's probably because 7th would be translated as zevende and not zeven when referring to ordinal numbers :)
You could try reporting it as an alternative, although I'm not exactly sure if it's correct - a native is probably better at explaining why it may/may not be accepted :)