Czech language translation fixes

[jsmucr]

cs.po appears to be mainly a product of machine translation, resulting into misinterpretations, terminology inconsistencies, and some hilarious translations in general. Some translations are missing, too.

Given the size of the file, this task will take some time to complete, so for now I'm marking it as a draft. I'll be adding commits as I continue using DT and getting acknowledged with its Czech version.