microkid

This plugin gives you full control over the permalinks for author pages. When using “pretty permalinks”, by default URLs to author pages are structured:
http://www.example.com/author/author-name/
With this plugin you can change the “author” part of these URLs into anything you like, for example:
http://www.example.com/editorial/author-name/ 
http://www.example.com/crew/author-name/ 
http://www.example.com/members/author-name/ 
http://www.example.com/posts-by/author-name/
The plugin will also automatically change links throughout your site that point to author pages. The URLs of author feeds are also changed.
Visit the <a href="http://www.microkid.net/wordpress/author-slug/" title="Wordpress Plugin: Author Slug" rel="nofollow ugc">plugin’s homepage</a> if you have any comments questions.

Custom Author URL

Customize the author slug in the permalink for author pages.

texttheater

NOTE: recent WordPress versions already properly transliterate umlauts if the 
site language is German. In that case, German Slugs is no longer needed.
While creating a post or page, WordPress automatically suggests a permalink. If 
your permalink structure contains the %postname% tag, then the title of your 
post or page will be used, simplified by conversion to lowercase, replacing 
spaces with hyphens, removing certain punctuation characters and removing 
diacritics from latin letters.
Thus, by default, ä is replaced with a, ö with o, ü with u and ß with s (similar 
for capital letters). For German, the conventional transliteration is ä to ae, ö 
to oe, ü to ue and ß to ss (sometimes sz in Austria, this is not yet supported). 
This plugin makes your WordPress apply the conventional transliteration.
I wrote this plugin because manually correcting WordPress’s suggestions is a 
considerable hassle, especially if you decide to change the title afterwards.
<h3>Support</h3>
If you have questions or suggestions, contact me at poststelle ät texttheater döt 
net.

German Slugs

German Slugs properly transliterates umlauts and the letter ß appearing in titles for slugs (i.e. for pretty permalinks).

Custom Author URL vs German Slugs