Small change to part 1 of German translation

Suggested by @HenningB.
Small rephrasings in part 2 of German translation

Suggested by @HenningB.
Quote identifiers in German translation

Suggested by @HenningB.
Uppercase "Typechecker" in German translation

Nouns are always uppercase in German, even if they come from English ;-)
Change quotes in part 3 of German translation

See 5110a3d - we should be consistent in the use of double, not single
quotes.
Fix English word in part 6 of German translation

Found by @HenningB.