Apparently 'stilsicher' translates as 'stylish' but I frequently see it used to describe one's proficiency in a language -e.g. 'Ihre Deutschkenntnisse sind stilsicher'. It just appears at first an odd word to use here - does it have any other added, or deeper, meaning other than 'stylish', and how, where, else is this word used?
"Stilsicher" means you are able to decide to which level of the language an expression, a word usage, a grammatical construction or a saying belongs, and that you use this properly in its right contexts. Language levels could be: high German, colloquial, slang, occupational jargon, poetic, dialect, children's language and many more.
If you are "stilsicher" enough you can also go outside of these boundaries, and use expressions that are out of place, but meant tounge-in-cheek, as a joke or ironically.
I'd say you don't usually comment on ones language skills with the expression "stilsicher". This is more of what could be requiered, to be able to manage some situations. It doesn't come naturally, only because you're a native speaker. It takes a lot of reading, writing and talking to aquire this.
I would translate "Stilsicher" as "style sure" or "sure of style," rather than "stylish."
"Sure of style" refers to a very high level of language proficiency, that even some native speakers can't attain. This is, you are not only "sure" or "sicher" regarding vocabulary, but also of style and nuances of the language. That is to say that you can make very subtle points in the language.
"Stylish" only means that you can "go with the crowd." Whereas "style sure" or Stilsicher suggests that you can rise above it.