While compiling the .mo files from the .po files with msgfmt 0.17, I get the following:
debian/squirrelmail-locales/usr/share/squirrelmail/locale/es_ES/LC_MESSAGES/serversidefilter.po: warning: Charset missing in header.
Message conversion to user's charset will not work.
debian/squirrelmail-locales/usr/share/squirrelmail/locale/tr_TR/LC_MESSAGES/squirrelmail.po:3173: a format specification for argument 2 doesn't exist in 'msgstr'
The second one is fatal and indicates a missing "%s" in the translation.