Minecraft Rescource Pack broke special characters

The only way to fix this (which it did) was to wait until the next day.

Disclaimer:
This joke was endorsed by Dinnerbone and Element Animation is (not) sorry.

This is a very long-standing bug in Minecraft on Mac OS X with so-called "dead keys" like Spanish keyboards' ´ and ` keys. (Your ñ works fine because it produces ñ using a single key instead of two.) Unfortunately, Mojang keeps thinking they have Minecraft's dead key handling fixed, but their Mac expertise seems to be insufficient and they don't seem to realise that fixing these keys on Windows doesn't automatically fix them on Mac OS X due to differences in how each operating system handles dead keys.

The most solid (but least quick) solution is to make noise on Mojang's bugtracker. Until Mojang fixes this on their end, there is no way to type these keys on a Mac OS X keyboard layout that uses dead keys.

A workaround is to manually design a new keyboard layout that doesn't use dead keys to type these letters, instead overwriting some of your other keys with keys dedicated to all the other accented characters (like the dedicated ñ key already is). You can do that using the Ukulele program in that link to create a custom layout. You'd have to sacrifice some keys, and which are up to you (I don't use Spanish keyboards and couldn't give useful advice for keys to replace), but the nice thing about Ukulele is that its graphical interface makes it easy to test and tweak your layout until it works for you.

Once you've created your layout and put the file in the right place (see Ukulele's instructions), you can use Mac OS X's Language & Text control panel to select your new keyboard layout as an keyboard option that you can easily switch to before launching Minecraft, then switch back to your regular Spanish layout when you're done. To make switching painless, make sure you have Show Input menu in menu bar selected in that control panel, so you can easily switch layouts from the OS X menu bar.