The po/de-CH.po file is a duplicate of po/de_CH.po.

The po/de_CH.po is the correct file to keep and po/de-CH.po should be dropped entirely.

The presence of po/de-CH.po leads to the installation of a /usr/share/locale/de-CH/LC_MESSAGES/publican.mo which is never going to be used because de-CH is not a valid ISO code for a language.

This has been detected by the Debian package checker tool lintian:

W: publican: unknown-locale-code de-CH
N:
N:   The package appears to ship locales for a language but uses an unknown
N:   locale code as a subdirectory of /usr/share/locale. This usually
N:   results in users of the intended target language not finding the
N:   locale. The language codes used in the locale directories are those
N:   from the ISO 639-1 and ISO 639-2 standards, not those usually used as
N:   TLDs (which are from the ISO 3166 standard).
N:   
N:   It is possible that the language code was mistyped or incorrectly
N:   guessed from the language's or country's name.
N:   
N:   Refer to http://www.loc.gov/standards/iso639-2/php/code_list.php for
N:   details.
N:   
N:   Severity: normal, Certainty: certain