good use of locale-settings (lang=, lc_...=)

hello,

i installed postgresql and finally got it to run. now i installed 
another (windows-)program and i came again to this very important 
question: how to properly use/set the locale-variables?

my newly installed program needs the (windows-)system-variable "lang" to 
be set to "de" (for me, so that i can read the menu-entries in my language).

i installed postgresql with the locale-setting "C".

now: will i run into troubles with my new-setted (and needed) "lang=de" var?

greetings
hans horwath
p.s.: is there a good summary for secure and long-term functioning 
locale-settings?

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