IIRC, Colossus' diction and vocabulary were pretty good. On the page, he would come across speaking fluent English. Did they have some method of indicating accents in the comics, like spelling out the Russian pronunciations? Did they indicate his origin by peppering his dialogue with a lot of Russian phrases or exclamations, like bozhe moi or calling everyone godspodin or tovarisch? The former is what you say they did with Nightcrawler, while the latter is common with French characters in books, where everyone is Monsieur this or Mademoiselle that, or Senor for Spanish, and so on.