enable parallel check of languages

Use parallel to spawn as many subprocesses as possible to accelerate
the tests.
Change Maruku check to fully functional mode

this is cleans up the code.
Force output of Rake messages to stderr
[cs] Typos and formulation changes in Ch. 1 up to 1.3 included.
Translating "Applying a Patch with am"
Reviewed "Distributed Git"
Reviewed "Distributed Git"
[pl] chapter 7 improvements
[pl] s/tą komendę/tę komendę/
Fix issue #620
[ko] update fonts and thanks for pdf
[ko] update ko/README
[ko] update translation
[ko] fix typo.
[ko] fix punctuations for figure
[de] Fix some incorrect comment syntax in german translation

- This makes it easier to use tools for automatic merge of original english
   text to german translation
[de] Chapter 2: Merge original english updates to german translation and translate them
[de] Chapter 3: Merge original english updates to german translation and translate them
[de] Chapter 4: Merge original english updates to german translation and translate them
[de] Chapter 5: Merge original english updates to german translation and translate them