Different audiences have different reactions to bad grammar. Financial professionals are put off by it, while teenagers expect it. Are you building ETrade, or MySpace? For something like ETrade, that's a handicap -- get an editor or a copywriter. For something like MySpace, you're fine.
Also, teenagers' bad grammar follows a certain pattern that is quite different than the "I don't speak the language fluently" pattern.
I am building project management application. I expect it to be used by freelancers and small companies. I guess it would be better if I hire someone.
I hope good copy-righter could help me to present the service in the right “light”.
You mean "copy-writer" as in "one who writes copy (text)". And I don't think you need someone else to write your copy. I think you just need someone to make small corrections to your copy. You need a "copy-editor".
I think it's important that the voice is yours, even if someone else helps with the grammar and/or spelling. Your English is clearly good enough to communicate what you intend to say. It just needs a little tidying.