Why does this \gappto cause "Missing \endcsname inserted"?

Question

I have a macro recoding the language:

\def\mylang{English}

and a command to convert the language name into its abbreviation:

\NewDocumentCommand{\StrToABBR}{m}{%
    \expandafter\lowercase{\IfStrEqCase{#1}}{%
        {english}{EN}%
    }%
}

Then there is a macro whose name ends with the abbreviation:

\def\mymacroEN{}

I would like to append some text to the last macro like this: \gappto{\csname mymacro\StrToABBR{\mylang}\endcsname}{...}. Of course some control to the expansion is needed, thus I added a few \expandafter:

\expandafter\gappto\expandafter{\csname mymacro\expandafter\StrToABBR\expandafter{\mylang}\endcsname}{Some English text}

However this causes the error Missing \endcsname inserted.

While writing the current question, I saw this question, and thought that this problem is probably caused by the use of \lowercase. However, even if I remove it and change \StrToABBR into

\NewDocumentCommand{\StrToABBR}{m}{%
    \IfStrEqCase{#1}{%
        {English}{EN}%
    }%
}

the error remains. What's wrong here and how can I fix this problem?

---

Below is a MWE.

\documentclass{article}

\usepackage{etoolbox,xstring}

\NewDocumentCommand{\StrToABBR}{m}{%
    \expandafter\lowercase{\IfStrEqCase{#1}}{%
        {english}{EN}%
    }%
}

% \NewDocumentCommand{\StrToABBR}{m}{%
%     \IfStrEqCase{#1}{%
%         {English}{EN}%
%     }%
% }

\def\mymacroEN{}
\def\mylang{English}

\begin{document}

% EN
\StrToABBR{English}
% EN
\expandafter\StrToABBR\expandafter{\mylang}

% Works: Some English text
% \expandafter\gappto\expandafter{\csname mymacroEN\endcsname}{Some English text}

% Doesn't work
\expandafter\gappto\expandafter{\csname mymacro\expandafter\StrToABBR\expandafter{\mylang}\endcsname}{Some English text}

\mymacroEN

\end{document}