If you source files contain non basic-latin characters, ASDF allows you to specify the encoding via the :encoding option of components (whole system, files, etc). UTF-8 is the recommended choice. To be compatible with old versions of ASDF use reader conditional #+asdf-unicode.
This way your source code will build independently on the locale.
The chapter about source file encoding in ASDF manual: http://common-lisp.net/project/asdf/asdf/Miscellaneous-additional-functionality.html#Miscellaneous-additional-functionality