Possible bugs in the markdown to PDF via lualatex conversion process for bilingual documents

["R (Chandra) Chandrasekhar" <chyavana-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org>]

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I am writing a bilingual test file for English and Tamil in the same 
markdown document to yield a PDF via lualatex and babel. The file, which 
correctly gives HTML and epub outputs,  is:

```
---
title: "Bilingual Test for LuaLaTeX and Babel: English and Tamil"
lang: en-GB
---

This is English.

{\sffamily This is also English.}

இது தமிழ்.

{\sffamily இதுவும் தமிழ்.}

:::{lang=ta}
இது கூட தமிழ் தான்.
:::

Back to English and EOF.
```

The included `babel-in-header.tex` file is:

```
% babel-in-header.textsf
\babelfont{rm}[Scale=1.0,Language=Default,BoldFont={* SemiBold}]{Noto Serif}
\babelfont{sf}[Language=Default,BoldFont={* SemiBold}]{Noto Sans}
\babelfont[tamil]{rm}[Language=Default]{Noto Serif Tamil}
\babelfont[tamil]{sf}[Language=Default]{Noto Sans Tamil}
\babelprovide[import,onchar=ids fonts]{tamil}
\defaultfontfeatures{Scale=MatchLowercase,Ligatures=TeX,Renderer=HarfBuzz}
\thispagestyle{empty}
```

The compilation command is:

pandoc --from markdown --to latex --include-in-header 
babel-in-header.tex --standalone --output en-ta-test.tex en-ta-test.md

I find that the generated .tex file cannot be compiled to PDF because 
there appear to be two errors, which might be bugs:

1. The \babelfont commands appear _before_ the \usepackage{babel} 
invocation in the .tex file because of a commented requirement for 
polyglossia, which I am not using. As a result, I get an

```
! Undefined control sequence.
l.59 \babelfont
              {rm}[Scale=1.0,Language=Default,BoldFont={* 
SemiBold}]{Noto Serif}
```

error with lualatex.

2. If I manually transpose the \babelfont commands inserted by the 
`babel-in-header.tex` file to appear _after_ the babel invocation but 
before \title in the .tex file, I get this error:

```
! Package babel Error: Unknown option 'tamil'. Either you misspelled it
(babel)                or the language definition file tamil.ldf was not 
found.


See the babel package documentation for explanation.
```

3. If I remove `tamil,` from the babel invocation,

```
\usepackage[tamil,main=british]{babel}
```

I am able to compile with lualatex, using the same command as before, 
and get the expected PDF, as shown in the attached screenshot.

I realize that both pandoc and the babel package are under development, 
and are therefore moving targets.

Nevertheless, I believe these are bugs in the markdown to latex 
conversion process and point them out so that they may be corrected or 
circumvented, automatically rather than manually, from the pandoc end.

Thanks.

Chandra

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