Matt Cutts, today said not to worry about comments with bad spelling & grammar left by others on your blog post in terms of page’s ranking abilities. In a video published on YouTube, however he said to make sure that the content written and comment left by the owner of the blog should not have such mistakes.
We often come in a situation where visitors who read our blog posts, leave comments which have spelling and grammar mistakes, may be some bloggers tend to give attention to these but mostly ignore such problems. You may have seen number of such comments on Youtube video website which does not make such sense. Matt said that these types of comments does not affect pages ranking, just make sure that these are not spammy or left by spamming bots.
You should take care to remove comments left by automatic comments bots OR the comments which are of spammy nature, basically as we know all types of user-generated spam can cause problems for page, mainly comments.
Do not assume that every visitor leaving comments will be Native English speaking, on web anyone seeking information can read your post and left the comments. So in reality it’s natural to have misspellings in comments. It is on you if you want to correct grammar, not approve such comments while these third-party comments will not have any negative effect on your search engine rankings.