Handling optional commas in text

Unobtrusive parts in a TeX file can be made with a single character if it is an active character. Here is how you can do it with ; for \optionalcomma (if you don't need real semicolons).

\documentclass{memoir}

\newcommand{\optionalcomma}{,}
% \newcommand{\optionalcomma}{}

\catcode`;=\active
\newcommand;{\optionalcomma}

\begin{document}
Well, what do you think; about this?

\end{document}

Piece of cake if you're free to use LuaLaTeX. :-)

The code below sets up two Lua functions, called functionA and functionB, and two LaTeX utility macros, called \OptionA and \OptionB to activate the respective Lua functions. By "activating" a Lua function, I mean assigning it to LuaTeX's process_input_buffer callback, which operates on the input at a very early stage, i.e., before TeX starts its usual macro expansion operations. Both functionA and functionB employ Lua's powerful gsub ("global substitution") string function.

I would assume that, in practice, your document will only contain either an \OptionA directive or an \OptionB directive. Ideally, you'd issue the directive immediately after \begin{document}.

The following screenshot shows the output of Han så,, at jeg kom, og at jeg var glad. if \OptionA is executed at the start of the document environment.

% !TEX TS-program = lualatex
\documentclass{memoir}
\usepackage[danish]{babel}

%% Set up two Lua functions:
\directlua{%
function functionA ( s )  return ( s:gsub(",," ,",") )  end
function functionB ( s )  return ( s:gsub(",," , "") )  end
}

%% Set up two LaTeX macros to activate the corresponding Lua functions:
\newcommand{\OptionA}{\directlua{luatexbase.add_to_callback(
   "process_input_buffer" , functionA , "functionA")}}
\newcommand{\OptionB}{\directlua{luatexbase.add_to_callback(
   "process_input_buffer" , functionB , "functionB")}}

\begin{document}
\OptionA % or: \OptionB
Han så,, at jeg kom, og at jeg var glad.
\end{document}

You might (ab)use \, for the purpose. Use \thinspace if you really need the original meaning of \, in text mode.

\DeclareRobustCommand{\,}{\ifmmode\mskip\thinmuskip\else\optionalcomma\fi}
\newcommand{\optionalcomma}{} % standard, no comma
\newcommand{\activateoptionalcomma}{\renewcommand{\optionalcomma}{,}}

Example with no optional comma

\documentclass{memoir}

\DeclareRobustCommand{\,}{\ifmmode\mskip\thinmuskip\else\optionalcomma\fi}
\newcommand{\optionalcomma}{} % standard, no comma
\newcommand{\activateoptionalcomma}{\renewcommand{\optionalcomma}{,}}

\begin{document}

Well, what do you think\, about this?

\end{document}

Example with optional comma activated

\documentclass{memoir}

\DeclareRobustCommand{\,}{\ifmmode\mskip\thinmuskip\else\optionalcomma\fi}
\newcommand{\optionalcomma}{} % standard, no comma
\newcommand{\activateoptionalcomma}{\renewcommand{\optionalcomma}{,}}

\activateoptionalcomma

\begin{document}

Well, what do you think\, about this?

\end{document}