(1) Yes, it's a setting in the System Preferences > International. By default, a whole number of languages is installed and you can even order by priority, just drag and drop the languages around. For instance, you can set it to (1) Spanish (2) English (3) German (4) etc.
If you download a program that doesn't have a Spanish localization, it automatically launches this app in English. If you were to put German to (2) and the app had only German and English as languages, it would use German instead.
You can change the language at any point. If you do, then all apps which are launched after the switch will respect your new settings. (This means, however, you need to log out and back in/force-quit the Finder to be sure.)
(2) No, but you can switch between different keyboard layouts at your leisure. On a US keyboard, option + n will give you `˜', so typing Option + n and then n again gives you ñ.
(3) Yes, if you tick the right check box in the Mouse & Keyboard preferences (in the System Prefs).
Edit: Everything I am telling you has worked since the Public Beta five years ago (well, with the exception that there was no Mighty Mouse; other three-button mice worked just fine, though).