ASDF encoding
The encoding used by lisp implementation to read source files is often depends on environment, for example the LANG environment variable.

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: