Passing american to \babelfont produces LaTeX error when realscript is used

[fiapps]

Explain the problem.
MWE (test.md):

---
lang: en-US
mainfont: cochineal
header-includes: | 
    ```{=latex}
    \usepackage{realscripts}
    ``` 
---

test^[this]

Command line: pandoc test.md --pdf-engine=xelatex -o test.pdf
Error:

Error producing PDF.
! LaTeX Error: Key 'fontspec-opentype/Language' accepts only a fixed set of
(LaTeX)        choices.

For immediate help type H <return>.
 ...                                              
                                                  
l.74 test\footnote{

Environment: pandoc 3.1.1, macOS 12.6.3, TeXLive 2023

The error depends on the line \babelfont[american]{rm}{cochineal}. If american is changed to english, xelatex completes the required two passes without error. With american, it fails on the second pass. Also if [american] is deleted, two XeLaTeX passes complete without error. I know 3.1.2 has been released, but it's not yet available through Homebrew, and the LaTeX template looks like it would still produce this line.

The bug here may be in babel, because specifying [american] works with TeXLive 2021, but I don't think pandoc should be passing any language here in the template. As I read the babel documentation, one passes a language to \babelfont in order to use a different font for a specific language. So this first invocation should not pass a language, because it's setting the default font. It's only with the new babelfonts variable that a language should be passed, to override the default \babelfont for a specific language or languages.

So I propose changing line 366 of data/templates/default.latex from
\babelfont[$babel-lang$]{rm}[$for(mainfontoptions)$$mainfontoptions$$sep$,$endfor$]{$mainfont$}
to
\babelfont{rm}[$for(mainfontoptions)$$mainfontoptions$$sep$,$endfor$]{$mainfont$}

[jgm]

You're probably right. We can try it. See #8538 -- I want to make sure we don't break what we fixed there.

[jgm]

OK, seems to work in both cases. I'll make the change.

[fiapps]

Thanks for the quick fix. I'll just use the corrected template with --template until the next release.