The usage of this word varies widely depending on the region of the country. (much better than "widely varies")
Should a comma be put before "depending"? (no "the" before "depending", and no "the" before region)
I also would not put a comma before "depending".
You need "the", not "a", before region. [usage varies depending the regions of the country, not just (one)(a) region] This is a little confusing; if you don't understand, please ask.