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.
(asdf:defsystem :my-system
:author "me"
:version "1"
#+asdf-unicode :encoding #+asdf-unicode :utf-8
:components ((:file "file-1")
(:file "file-2")
...
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
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