Yes. Just an artefact of the text-to-speech software used by Duo. It is pretty good but not perfect, as explained in the course notes from time to time.
Deall is a verb-noun, the basic form of verbs in Welsh and some other related languages. These can be translated as a form such as 'to understand' and also as a verbal noun such as 'understanding'.
In the welsh course I'm doing we were taught "dallt", is there a difference between "dallt" and "deall"? Is it a north/south dialect thing?
dallt reflects the pronunciation and variation in the colloquial language in north-west Wales, as far as I know. There are some other variants, too, such as dyallt, deallt.
Since "Dw i'n deall" means "I understand", wouldn't it be OK to translate this as "understand"?
That would leave dw i'n to mean 'I', when in fact just i means 'I' there.
Word-by-word translations of sentences do not often work between Welsh and English - the languages have very different structures.
The nearest literal English meaning of deall on its own is the participle 'understanding', although it is often also rendered as the infinitive 'to understand'. In a dictionary you would look up 'understand', but Duo is not a dictionary, so we cannot include the dictionary notes that would explain deall as being a verb-noun and not a verb, and so avoid the confusion with the command form 'understand!', etc.
This is explained somewhere in one of the earlier sets of course notes.