\@ifpackagelater equivalent for babel's lan­guage def­i­ni­tion files?

The babel language files use \ProvidesFile with some version information argument -- this setup is necessary to use the LaTeX core macro \@ifl@ter to check the file date, if it is applied with the correct extension.

It checks the file date, see the definition and the usage in latex.ltx:

\def\@ifl@ter#1#2{%
  \expandafter\@ifl@t@r
    \csname ver@#2.#1\endcsname}
\@onlypreamble\@ifl@ter
\def\@ifl@t@r#1#2{%
  \ifnum\expandafter\@parse@version#1//00\@nil<%
        \expandafter\@parse@version#2//00\@nil
    \expandafter\@secondoftwo
  \else
    \expandafter\@firstoftwo
  \fi}
\@onlypreamble\@ifl@t@r
\def\@parse@version#1/#2/#3#4#5\@nil{#1#2#3#4 }
\@onlypreamble\@parse@version
% Some lines omitted
\def\@ifpackagelater{\@ifl@ter\@pkgextension}
\def\@ifclasslater{\@ifl@ter\@clsextension}
\@onlypreamble\@ifpackagelater
\@onlypreamble\@ifclasslater

As can be seen, the macros \@ifpackagelater and \@ifclasslater do not have arguments actually (they are moving!) and the final treatment of the {true}{false} branches is left over to \@ifl@t@r.

So all that is to the is to define a separate macro that checks for a babel definition file, say \@ifbabellater and provide the \@b@belextension here.

\documentclass[english]{article}

\makeatletter
\def\@b@belextension{ldf}
\def\@ifbabellater{%
  \@ifl@ter\@b@belextension%
}
\makeatother
\usepackage{babel}
\makeatletter
\@ifbabellater{english}{2012/08/20}{%
  \typeout{After}
}{%
  \typeout{Before}
}
\makeatother
\begin{document}
\end{document}

In my setup it prints 'After' on the console.

Apparently, there's a package for such macros, but I've done with core methods ;-)

In LaTeX, Version dates can be declared by:

\ProvidesClass{<class name>}[YYYY/MM/DD ...]
\ProvidesPackage{<package name>}[YYYY/MM/DD ...]
\ProvidesFile{<file name>}[YYYY/MM/DD ...]

and LaTeX provides:

\@ifclasslater
\@ifpackagelater

\@iffilelater is missing and both commands can only be used in the preamble.

Package ltxcmds closes the gap by defining the complete set:

\ltx@ifclasslater
\ltx@ifpackagelater
\ltx@iffilelater

All three commands can also be used after \begin{document}.

Example:

\documentclass[english]{article}
\usepackage{babel}
\usepackage{ltxcmds}
\makeatletter
\ltx@iffilelater{english.ldf}{2012/08/20}{%
  \typeout{After}
}{%
  \typeout{Before}
}
\makeatother
\begin{document}
\end{document}