Bug in iflang.sty

[eg9]

The iflang package decides whether the format is based on language.dat or etex.src based on whether \uselanguage is defined.

Unfortunately, beamer defines \uselanguage, so the package sets \IfLang@prefix to lang@ instead of l@ even if the format is based on language.dat. As a consequence the tests which use \IfLang@prefix will produce the wrong result, including the final \IfLanguagePatterns{\languagename} test at the end of package loading.

Proposed fix: use an internal command defined in etex.src rather than \uselanguage.

Reference: https://tex.stackexchange.com/q/411254/4427