ThankYou for your answers. They help me to understand Linux a little bit
more and solving my problem :)
I searched for an english mo-file and I didn´t find some. So I think,
your solution is the best, Simon, because it sets the locale to the
native language of the application, in this case to english, I think.
I like it very much to play with Linux. So I tested other locales for
"LANG" and find out, that it falls back to native written language, if
the locale doesn´t exist. For example: darktable-"en" doesn´t exist. So
"LANG=en darktable" will fall back to the native language of the
application. "LANG=rrrr darktable" does the same and "LANG= darktable"
starts darktable in english too.
I changed the run-string in the desktop-file for starting darktable from
"darktable %U" to "LANG=C darktable %U" and now it starts in english.
Edgar