Two important things in usability that are often overlooked are spelling and grammar. If users have problems following your sentences or recognizing the words you are using for important user interface elements (and not every user is a native speaker, and though they may be good at a language, they may still have problems recognizing a sentence’s parts if one word misleads them), they will have issues using your application.
Moreover, if your spelling and grammar are shoddy, this reflects badly on the rest of the app: If they couldn’t even bother to make sure the writing is OK, how must the code behind it look?
My sister just sent me a link to an article that illustrates with numerous examples that running the spell checker over your text will miss many mistakes:
You will want to go there and read it. There’s always one word that one wasn’t aware of was actually spelled differently.