# Situation

I need to set a few commands and variables based on what language a document is written in. To do this, I pass an ISO 639-1 language code to a command (\DeclareRobustCommand{\setlanguage}[1]. This variable is then sent to other subcommands contained by this e.g. \renewcommand{\dq}[1]. I do not want this to occur. How can I make the subcommands receive their inputs from where they are called in the document?

# Problem

The problem is that the renewal of the \dq command (a subcommand of \setlanguage) receives the input of its containing command (e.g. "en") and not of the text (e.g. "black hole").

# Minimal Working Example

\documentclass{article}
\usepackage{fontspec}
\usepackage{xstring}

\DeclareRobustCommand{\setlanguage}[1]{%
%    \IfEq{#1}{en}{%
\protect\renewcommand{\dq}[1]{\char"201C{}#1\char"201D{}} % Set double quotes to standard in respective language
\protect\renewcommand{\sq}[1]{\char"2018{}#1\char"2019{}} % Set single quotes to standard in respective language
%    }{}%
}%

\begin{document}
\setlanguage{en}

Although the term \dq{black hole} was not coined until 1967 by Princeton physicist John Wheeler, the idea of an object in space so massive and dense that light could not escape it has been around for centuries. Most famously, black holes were predicted by Einstein's theory of general relativity, which showed that when a massive star dies, it leaves behind a small, dense remnant core. If the core's mass is more than about three times the mass of the Sun, the equations showed, the force of gravity overwhelms all other forces and produces a black hole.

\end{document}

• Use ##1 inside the nested renew. – Steven B. Segletes Mar 23 '15 at 14:31
• @steven-b-segletes Make it an answer and you've got it! – Jonathan Komar Mar 23 '15 at 14:53
• Related: tex.stackexchange.com/questions/232636/… ... and it has nothing to do with \DeclareRobustCommand – user31729 Mar 23 '15 at 15:35
• Side comments: \DeclareRobustCommands are not expandable, but this is no issue for this case, I suppose and the quotes might be easier achieved with csquotes (at least in a non xelatex - environment) – user31729 Mar 23 '15 at 15:43

The # sign followed by a single digit (1-9) is used to indicate an argument number to a macro. Thus, the way the OP's MWE was structured, an invocation of #1 inside the macro \setlanguage is interpreted as the first argument of the \setlanguage macro, which was specified without arguments. Thus, it throws an error.

LaTeX provides the means of "doubling" the # character (##) as the means to convey a single # through the expansion/replacement. Thus, ##1 is not perceived by \setlanguage as its first argument, but is instead replaced by #1 for the interpretation by the embedded macro \dq.

One can determine the rule for recursion here: for each layer of \def or \(re)newcommand inside of a macro definition, the number of # characters need doubling:

Layer  Argument
1      #1
2      ##1
3     ####1


etc. At the end of the revised MWE, I whow an example with triple-nested arguments.

\documentclass{article}
\usepackage{fontspec}
\usepackage{xstring}

\DeclareRobustCommand{\setlanguage}[1]{%
%    \IfEq{#1}{en}{%
\protect\renewcommand{\dq}[1]{\char"201C{}##1\char"201D{}} % Set double quotes to standard in respective language
\protect\renewcommand{\sq}[1]{\char"2018{}##1\char"2019{}} % Set single quotes to standard in respective language
%    }{}%
}%

\begin{document}
\setlanguage{en}

Although the term \dq{black hole} was not coined until 1967 by Princeton physicist John Wheeler, the idea of an object in space so massive and dense that light could not escape it has been around for centuries. Most famously, black holes were predicted by Einstein's theory of general relativity, which showed that when a massive star dies, it leaves behind a small, dense remnant core. If the core's mass is more than about three times the mass of the Sun, the equations showed, the force of gravity overwhelms all other forces and produces a black hole.

\def\first{F\def\second##1{(S##1)\def\third####1{[T####1]}}}

\first, \second{x}, \third{y}

\end{document}