How to typeset text with bilingual fragments with LaTeX

This post gives a possible solution for typesetting two similar versions of the same LaTeX document differing only in some text fragments.

Here is the code:

\documentclass{article}

\makeatletter
\newif\ifsecondlanguage
\def\@bracedfirst#1#2{{#1}}
\def\@bracedsecond#1#2{{#2}}

\def\do@localized#1[#2]{%
  \let\@@localized\@undefined
  \newcommand\@@localized[#2]{#1}\@@localized}%

%** Typeset different text depending on the current value of \ifsecondlanguage.
%** Usage: \bilingual{LANG1PAT}{LANG2PAT}[ARGCOUNT]{ARG1}{ARG2}...
%** If \ifsecondlanguage is true, pattern LANG2PAT is used, else LANG1PAT is
%** used. Arguments #1, #2 etc. in the pattern are substituted with
%** ARG1, ARG2 etc.
\def\bilingual{%
  \expandafter\expandafter\expandafter\do@localized\csname
    @braced\ifsecondlanguage second\else first\fi\endcsname
}

\begin{document}

\secondlanguagetrue
\bilingual
  {This is addition: #1; and this is multiplication: #2.}
  {Translation to another language: #1 and another translation: #2.}
  [2]{$x+y$}{$x*y$}

\secondlanguagefalse
\bilingual
  {These are additions: #1, #2; and this is multiplication: #3.}
  {Translation to another language: #1, #2 and another translation: #3.}
  [3]{$x+y$}{$x\oplus y$}{$x*y$}

\end{document}

Select the language to use with \secondlanguagetrue or \secondlanguagefalse. Whenever you have a text fragment which needs to be typeset differently depending on the languagem, use the \bilingual command, as demonstrated above.