It's definitely Twain & German, but it may be "drink" not "beer".  I remember reading it recently.  Based on what I've been reading lately I think it's in his (Cal. edition) notebook written while he was on his _Tramp Abroad_ tour.

larry marshburne              [log in to unmask]

>>> Stephen Railton <[log in to unmask]> 01/10/00 12:36PM >>>
 'I would rather decline a beer than a
Latin noun.'  Could you verify/correct this quote?
        It rings a bell (though I wonder if it wasn't about
German rather than Latin?) but I can't say for sure if it's
the real McTwain.