How can we tell whether it's 'flydende' is an adjective (and so 'fluent Spanish') or whether it is an adverb (as in 'speaks fluently')?
I can't. I just looked it up. http://ordnet.dk/ddo/ordbog?query=flydende&search=S%C3%B8g
It is created from the verb: flyde (float), the 'Spanish' language is floating (on her tongue). .
Or that the Spanish flows freely from her.
That is how I always understood it. Flyde also means flow.
That is a realy good question.
Normaly danish verbs made into adverbs end in -t and english adverbs end in -ly
But flydende does not have a -t form so no help there
And both she speaks spanish fluently and she speaks fluent spanish translates to hun taler flydende dansk
Adverb, you can also say 'hun taler spansk flydende', which would translate to 'she speaks spanish fluently
I think DL English 'always' uses 'speak' together with a certain language, 'speak English' etc.