Allow to set LANG and LC_ALL in envvars

Some apps/modules like mod_dav and mod_wsgi need the language to be set to specific values. Lets allow that through envvars (on debian systems only?).

This should be backward-compatible as 'C' remains the default. If a language is set, it is used, if the parameter is set to 'system', /etc/default/locale gets importet.
Because I like `.fetch` :-)

Given that it is currently only used in debian, I felt it more appropriate not to define it in the attributes. When its used for other platform_families too, it should be in attributes, yes.