Have found the most definitive answer to Twain's opinion of the German language
to be the article that appears in Rasmussen's _Mark Twain A to Z_.  There is
a rather lengthy entry on this topic. (In fact, I have found that the
_A to Z_ volume appears to answer about 95% of most Twain questions that
I could have dreamed up to submit to this forum.)
 
One other source for Twain and German that I recall is Clara's book
_My Husband, Gabrilowitsch_.  A view straight from someone within the family--
don't have the passage in front of me or I'd post it.
 
Barb