Came here looking for the same thing as Deteros, but it's no big deal for me: I'm not German, I just write it sometimes, so a dictionary is all I need. Thanks to C0D312 for pointing me to one!
Also I felt I should clear up a few things:
These make sense as part of localisation.
These already exist. Hit the usual key, and your ü, ä, ö or ß will appear.
You might have problems on Linux filesystems with non-ASCII characters in filenames, but that's a problem with Linux filesystems, not Sublime Text.
The dictionary you'll have to install yourself. It'd be nice if ST came with a package for installing new dictionaries, but distributing dozens of large dictionary files with the app just in case somebody needs one is too much.
These don't make any sense. Sublime Text is a code editor, not a spreadsheet or word processor. Formatting numbers is up to the user, not the software, and adding German "smart quotes" is bad in a code editor: it'd screw up the code.
That'd be the domain of (programming/markup) language-specific Packages (e.g. they might make sense in the Markdown package, and it would be up to that package to use German "smart quotes" („“) instead of English ones (“”) if the language were set to German.