Xindy seems to ignore sort-rules in custom .xdy file

Question

So I am trying the change the way xindy is sorting umlauts in an index file by prescribing custom sort-rules.

This is the myfile.xdy that I use:

(require "lang/german/duden-utf8-lang.xdy")

(sort-rule "Ä" "Ae")
(sort-rule "Ö" "Oe")
(sort-rule "Ü" "Ue")

(sort-rule "ä" "ae")
(sort-rule "ö" "oe")
(sort-rule "ü" "ue")

And here is a minimal example for a document:

\documentclass[]{article}
\usepackage{fontspec}
\usepackage[ngerman]{babel}

\usepackage[xindy, splitindex]{imakeidx}

\def\xindylangopt{-M lang/german/myfile}
\makeindex[name=test, options=\xindylangopt]



\begin{document}
    foo \index[test]{Aal}  \index[test]{Aeal}  \index[test]{Äal} \index[test]{Abal} \index[test]{Afal}   
    \index[test]{Ual}  \index[test]{Ueal}  \index[test]{Üal} \index[test]{Ubal} \index[test]{Ufal} 
    \index[test]{Oal}  \index[test]{Oeal}  \index[test]{Öal} \index[test]{Obal} \index[test]{Ofal} 

    \printindex[test]
\end{document}

This is what the output looks like:

But this is still the original sorting where the umlauts are treated as if they were their respective vocal, i.e. 'Ä' is treated as 'A' and so on. What I would like is that they are treated as their respective vocal followed by the letter 'e', i.e. 'Äal' would be placed between 'Aeal' and 'Afal' in this example. What am I doing wrong? Can the sorting rules not be overwriten like that? Do I even need a custom file for this or are there already native modules that do this kind of sorting?

Answer 1

Thanks to @Sergei Golovan, a solution was found. It appears to be sufficient to use the din5007-utf8-lang.xdy-module instead of the duden-utf8-lang.xdy-module as a basis, since the din5007-utf8-lang.xdy-module already sorts the umlauts the way that I was trying to achive. So in case other changes should still be applied to the sorting, the required module needs to be adjusted.

myfile.xdy:

(require "lang/german/din5007-utf8-lang.xdy")

some other custom sort-rules

If no other changes to the sorting need to be applied, it is sufficient to change the xindy language options in the document like so:

\def\xindylangopt{-M lang/german/din5007-utf8-lang}