ICU package contains a special code "mul" with a self-name of "multiple
languages". libkiwix now suppresses that. As a result the self-name of
"mul" (like for any other unknown language code) is the code itself
(i.e. "mul").
The most prominent user-visible effect of this change is that the
language filter in the library page no longer contains a "Multiple
languages" entry if there is a legacy ZIM file with the language set to
"mul" - that entry now shows up as "Mul".