another common usage is to standardize error/success/etc messages. you always want one type of error to say the same thing, and a language file is a better place to put things than using a define.

having said that, its all about maximizing efficiency. if you don't think you'll actually save that much time in the future by putting things in the language file, maybe because you'll "never" internationalize, then its probably a process worth skipping for you. you can always just go back and make the changes later. it'll just be harder and take longer to do it later, as opposed to doing it now.