bilingual slides: beamer, comment package and non-ascii characters (umlauts, diacritical characters)

Question

I'd like to generate beamer slides matching the main language of the target audience. The easiest way seems to facilitate the comment package and pack the individual slides into includecomment/excludecomment environments... (as suggested in a previous question).

However this breaks in various ways when non-ascii characters are involved depending on the location and character (and even document class) used, for example:

• ./comment.cut:2: Package inputenc Error: Keyboard character used is undefined
• ./comment.cut:1: Argument of \UTFviii@three@octets has an extra }.
• ./comment.cut:3: Package inputenc Error: Unicode char \Uffffffffifnum (U+2E7) not set up for use with LaTeX.

How can I make the following MWE work and what's the underlying issue?

\documentclass{beamer}

\newcommand{\lvlang}{DE}

\usepackage[utf8]{inputenc}
\usepackage[T1]{fontenc}

\usepackage{comment}
\usepackage{ifthen}
\newcommand{\iflvlangde}[2]{%
  \ifthenelse{\equal{\lvlang}{DE}}{#1}{#2}%
}

\makeatletter
\iflvlangde{
  \usepackage[main=ngerman]{babel}
  \includecomment{DE}
  \excludecomment{EN}
}{
  \usepackage[main=english]{babel}
  \includecomment{EN}
  \excludecomment{DE}
}
\makeatother

\begin{document}
\begin{DE}
\begin{frame}[fragile]{mäh}
  föö
\end{frame}
\end{DE}

\end{document}

Answer 1

As I have found out while writing up the question, the issue is the comment package or more specifically that it writes out expanded characters to its intermediate file. See the original answer for more details.

The solution is to patch comment's WriteCommentLine and ThisComment commands by adding the following after loading comment:

\long\def\WriteCommentLine#1{\immediate\write\CommentStream{\unexpanded{#1}}}
\let\ThisComment\WriteCommentLine

Thanks egreg!