Dark's spells

Well, many Japanese (and as such, many potential Anime recipients, who are the calculation basis for Anime producers) are germanophile to some degree. This may be an aftereffect of the alliance during WW II and/or caused by the generally good international relations between Germany and Japan. It does work vice versa too, after all: Many people in the German-speaking part of Middle Europe have an affinity of some sorts for Japanese food, language, culture etc. (and yes, the still ongoing Anime Hype starting in the late 90s certainly has something to do with this as well.)
So, it does seem reasonable enough to see German words and expressions (however inadequately used) popping up in Anime every once in a while. The grammar is usually questionable at best due to the fact that Japanese and German are vastly different languages, I presume (which probably is also one of the key problems defining the phenomenon known as "Engrish"). Japanese is an agglutinative language while German is mostly synthetic, word order and sentence structure are different, Japanese uses a syllabic script while German uses an alphabetic script, etc.
Alas, German is quite hard to learn for someone who doesn't have it as their native language (or so I've heard), and being Japanese- thus coming from an entirely different cultural and linguistic background- certainly won't exactly help, either. So, mistakes are understandable (and plentiful) on such occasions.
While Ancient Greek might be a nice-sounding alternative for incantations and all that jazz, I suppose Japanese would have an even harder time getting a grasp of it than they already have with the German language. Not to mention that Ancient Greek is passed down mostly by using Greek letters (duh), which would be an additional challenge for our poor Anime producers- with German, they at least already know the letters, since Japanese certainly use Romaji often enough.

"We spread the time as we can, but in the end the world takes it all back." [Roland Deschain]