query: mixing natlangs

Is it at all wise to attempt to mix together two natlangs, one of which
you've only had a year of instruction in, the other of which you only
have two grammars of, if you are *not* attempting a very
naturalistic-looking result?  I have been playing with a Japanese-German
mix (German lexis mangled mostly toward Japanese phonology with
Japanese/Korean-style grammar), but I have yet to actually learn more
than phrases and basic grammatical factoids about Japanese.

(As a side note, one of the reasons I tend to skim conromancelang posts
is that, being only familiar with French and the beginnings of Latin, I
get confused way too easily.  I feel slightly guilty, but I'm taking it
one language at a time....)

And related to that--if anyone would be willing to recommend general
reference books on (high?) German and Japanese, specifically with regards
to (changing modern?) phonology/phonetics, I would be grateful.

YHL