'Well' is not an adjective, and saying "you speak well Spanish" doesn't make sense. The Spanish that is spoken cannot 'be well'. It is an adverb though, so you can say "You speak Spanish well". That means that the way that the Spanish is spoken is well, which is perfectly acceptable.
In several cases I get confused by the way you translate phrases, in regard with the presence of the article "a" implicitly.
Here for example I translated with "You speak a good Spanish", and you mark it as wrong cause there is no article "en"/"et" specified. Right. Fact is that in many other cases the article is not present, but you mark a translation without it as wrong.
Are there rules for the insertion or not of an indefinite article in a phrase?
According to the Wiktionary page 'spansk' when it refers to the language is masculine. That said, the gender of the word 'spansk' has nothing to do with why it's 'godt.' It's 'godt' because it's an adverb here and not an adjective. It's modifying the way you speak and not the language that you speak, and when it's an adverb it's always 'godt.'