Translation:I am walking as slow as a snail.
It also translates as 'slowly' and this is something which needs explained:
To make an adjective into an adverb in Gaelic, you add gu unless the adjective is preceded by some small word such as cho 'as', glè 'very', nas to make a comparative, etc. So cho slaodach is the standard Gaelic for both 'as slow' and 'as slowly'.
It is Duolingo policy to accept the the variety of ways that different people speak. Since, as you say yourself, slow is common colloquially, that makes it acceptable by definition. We are very lucky in English not to have any government body telling us what is correct, so just because some writer of school text books said we should say slowly that does not mean we actually have to do that.
From the point of view of historical linguistics, there are loads of examples where colloquial English is closer to the Celtic languages than standard English is. This is no surprise given than most of our ancestors were Celtic speakers, so this is simply another situation where the colloquial English agrees with the the Gaelic. I think Gaelic has had the most influence on English in the last 400 years, although Welsh was more influential before that.
That simply doesn't work, as an example like this shows: there's no end to the different ways people speak. A language learning site needs to use standard, correct English: that's the only way learners can know where they are. It can be extremely irritating and frustrating to have a translation 'marked' wrong only because the course has used English slang or dialect.
There is. It is fast. There is no different adverbial form. It is important to distinguish between what exists and is unmarked like this, and what doesn't exist. Quick and fast can be made into adverbs but old can't
The quick car was goingquickly
The fast car was going fast
*The old car was going