[Tex/LaTex] “Argument of \reserved@a has an extra }” error when using non-English language in \title

Question

(Other answers (A, B) with similar error message do not actually take care of the situation.)

I have memoir document which should contain title both in Bangla and English. But when compiled (using xelatex), I get an error message,

! Argument of \reserved@a has an
extra }.

Here is the MWE which causes the error.

\documentclass{memoir}

\usepackage{polyglossia}
\setmainlanguage{english}
\setotherlanguage[numerals=Devanagari]{bengali}

\title{\begin{bengali} বইয়ের টাইটেল\end{bengali}
  Book Title}

\begin{document}

\maketitle

\end{document}

If the part \begin{bengali} বইয়ের টাইটেল\end{bengali} is taken out of the file,
the error disappears. So, that line must the culprit here.

As is advised in the questions (or answers) mentioned at the top, I need to protect something. However, I am a little confused what to protect here. I even tried putting \protect around \begin{bengali} বইয়ের টাইটেল\end{bengali}, without any avail. (Just a hunch, not an educated one.)

Answer 1

The \begin macro which starts an environment is "fragile", which means it cannot be used inside a moving argument. The \title command expands its argument, and therefore fragile commands can't be used inside it.

See the following question for an explanation of this.

• What is the difference between Fragile and Robust commands?

So the solution is to not use the {bengali} environment inside the title.

(You could actually "protect" the environment by using \protect\begin{bengali} .... \protect\end{bengali}) but there is a simpler solution:

Just use \textbengali{}:

\documentclass{memoir}

\usepackage{polyglossia}
\setmainlanguage{english}
\setotherlanguage[numerals=Devanagari]{bengali}
\newfontfamily\bengalifont[]{Bangla MN}
\title{\textbengali{বইয়ের টাইটেল}
  Book Title}

\begin{document}

\maketitle
\end{document}