The page can be any language you like. I thought you were asking about the URL. Or are you asking about the URL?
Do the directory names have to have accents? For example are they based on people's personal names?
If you simply need to convert your õ's and ñ's into something else, your php script should be able to do that in a few lines. The tricky part is making something that converts in both directions. ñ > nh > ñ is no problem, but you can't simply change ã to am or an and then change "back" again.
Technically there is no rule against using non-ASCII characters in an URL or in any kind of link. It's just not always safe. I once had to change a bunch of fragment links that used yogh. They were safe on my HD and safe in my personal www space-- but all it takes is one server that doesn't use UTF-8 and it would have been a horrible mess.