There are actually Wiktionaries for many languages and the English Wiktionary also defines and translates words of many other languages. But ignoring these other languages, have you tried the English Wiktionary and do you find it to be a good writers' resource? What are its best and worst points in this regard?
I've tried the Wiktionary and I really like some aspects of it, like having all the different regionalisms listed next to each other. (eg. 'fanny' has both the benign North American definition and the less-innocent UK version).
But sometimes I go to a dictionary in need of an authority. You know... 'Webster's says..." or "According to the OED...", and the Wiktionary doesn't serve too well for that purpose.
So, for me, it's a good resource if I'm looking for basic word meanings for my own understanding, but it doesn't work as well if I'm trying to convince someone else of what a word means.
Here's the perspective of an editor who does some writing on the side:
It depends on what you need in a dictionary. When editing UK writers, I usually use Cambridge, I think I'd continue to use that or Merriam-Webster when trying to convince a writer, I do not think the word you used means what you think it means.
Technical writers or those in other specialized fields might want a different dictionary altogether. Academic writers should be using whatever dictionary their institution favors. If you're writing or editing to a particular style guide, that guide will have a preferred dictionary. (For example. AP uses Webster, Chicago is fine with either Webster or Merriam-Webster.)
As to how useable it is for everyday work, I'd rather use Cambridge or Webster. Both have easier-to-use interfaces, and Wikitionary often feels a bit like overkill to me. It tends to dwell on etymology and sourcing a bit too much to be practical for what I do. It might be a good place to get the general idea of a word, similar to how Wikipedia is a good place to go to start research on an unfamiliar topic.
In summary, while I wouldn't use it as an authority, but it has a place in my Editing > Dictionaries bookmarks folder.