CONTRIBUTING.md: Bug report with LANG=C (#9890)

* CONTRIBUTING.md: Bug report with LANG=C

* Apply suggestion

Co-authored-by: Sylvestre Ledru <[email protected]>

---------

Co-authored-by: Sylvestre Ledru <[email protected]>

Author: oech3

CONTRIBUTING.md

@@ -87,6 +87,7 @@ are some tips for writing good issues:
   - What platform are you on?
 - Provide a way to reliably reproduce the issue.
 - Be as specific as possible!
+- Please provide the output with LANG=C, except for locale-related bugs.
 
 ### Writing Documentation