[NTG-context] Some localizations

Hello Hans, quite incidentally I've found out that some of the localization files (lang-*.tex) are incomplete. I've filled in missing phrases in lang-ger.tex and lang-sla.tex for the languages I speak, i.e. English, German, Czech and Slovak. Please look at the attached files and if you consider them OK (they really should be! :-) ), please add them to the distribution. Thank you! Richard