The translation without "the" would be incorrect. "Teeth are in mouth" just wouldn't be "good English," although the meaning would be understood.
And what about article "a". Why can not i use article "a"? That mouth is from someone specific?
From the English side, seems to me that "Teeth are in A mouth" should also be okay.
If you used "a" and got it wrong, you could keep practicing the skill this sentence is in and wait until you get the sentence again, then use "a" again. If it's still considered incorrect, you could report it as "My answer should be accepted," Maybe DL will then consider it as an alternative translation.
"Teeth are in mouths" was not accepted. I think that "Teeth are in the mouth" would be a more common statement in English, but apparently in Czech the plural of "ústa" is used