Set LANG=en in parse_git_dirty

[hennes-maertins]

The function parse_git_dirty needs the output of git status to be in
english. This means to lose information in the git status of your prompt if
git speaks to you in a different language.

For example a git status output could be the following in german:

Auf Branch master
Ihr Branch ist auf dem selben Stand wie 'origin/master'.
zum Commit vorgemerkte Änderungen:
  (benutzen Sie "git reset HEAD <Datei>..." zum Entfernen aus der Staging-Area)

geändert:       js/easyprompt.js

To force git to speak english you can set LANG=en in the function
parse_git_dirty before git status is called. This modification of LANG
is just local. The user will still get the output in his language when he calls
git status by himself.