Expandable case insensitive switch case for string comparison

Question

I have spent the last few hours trying to come up with a good solution for my problem, but alas all solutions falls short. I want to create a switchcase for a function (In reality I need many different kinds, not just one for language, but I added it as an concrete example.)

My ideal switch case function has the following functionality

• It accepts commands as inputs (fully expandable)
• It is case insensitive

My code below is a switch case, but it fails both of the above requirements. Any help figuring out a proper way to do this is more than welcome.

\documentclass{article}

\usepackage[norsk,nynorsk,british,samin]{babel}
\usepackage{pdftexcmds}

% This allows the user to switch languages using a myriad of synonyms
% The four provided languages are: English, Norsk, Nynorsk and Samin
\makeatletter
\newcommand*{\selectLang}[1]{%
  \stringcases%
  {#1}%
  {%
    % English synonyms
    {english}{\selectlanguage{british}}%
    %
    % Norsk (Bokmål) synonyms
    {norsk}{\selectlanguage{norsk}}%
    %
    % Nynorsk synonyms
    {nynorsk}{\selectlanguage{nynorsk}}%
    %
    % Samin synonyms
    {samisk}{\selectlanguage{samin}}%
    {samin}{\selectlanguage{samin}}%
  }%
  {}%
}

% Helper functions to create the switch case above
\newcommand{\stringcases}[3]{%
  \romannumeral
  \str@case{#1}#2{#1}{#3}\q@stop
}
\newcommand{\str@case}[3]{%
  \ifnum\pdf@strcmp{\unexpanded{#1}}{\unexpanded{#2}}=\z@
    \expandafter\@firstoftwo
  \else
    \expandafter\@secondoftwo
  \fi
  {\str@case@end{#3}}
  {\str@case{#1}}%
}
\newcommand{\str@case@end}{}
\long\def\str@case@end#1#2\q@stop{\z@#1}
\makeatother

\newcommand{\temptwo}{nynorsk}

\begin{document}

\newcommand{\temp}{\temptwo}

\languagename

\selectLang{english}

\languagename % Displays english as it should

\selectLang{Norsk}

\languagename % This should display norsk

\selectLang{\temp}

\languagename % This should display nynorsk

\end{document}

Answer 1

With the kind help of expl3, this is expandable (if the code of the cases is, so with the normal definition of \selectlanguage it isn't).

EDIT: as pointed out by @JosephWright, \str_foldcase:e is to be used for this kind of task. As a general rule of thumb: For processing use \str_foldcase:n, for text which should be printed out use \str_lowercase:n.

\documentclass[]{article}

\usepackage{xparse}
\ExplSyntaxOn
\NewDocumentCommand \selectLang { m }
  {
    \str_case_e:nn { \str_foldcase:e { #1 } }
      {
        { english } { \selectlanguage{british} }
        { norsk }   { \selectlanguage{norsk} }
        { nynorsk } { \selectlanguage{nynorsk} }
        { samisk }  { \selectlanguage{samisk} }
        { samin }   { \selectlanguage{samin} }
      }
  }
\cs_generate_variant:Nn \str_foldcase:n { e }
\ExplSyntaxOff

\renewcommand\selectlanguage{} % just to show that it works

\begin{document}
Chosen language: \selectLang{EnGlIsH}

\def\foo{NOrsK}
Chosen language: \selectLang{\foo}
\end{document}