PDF render with lang: de-CH fails when hyphen-german is not installed #15027

When rendering to PDF with lang: de-CH on a TinyTeX that has no German hyphenation patterns, Quarto installs babel-german but the compilation still fails, and hyphen-german is never installed.

First reported at rstudio/tinytex#515.

Reproduction

TinyTeX without hyphen-german and babel-german (fresh install, or after tinytex::tlmgr_remove()):

---
title: "Untitled"
lang: de-CH
format: pdf
---

Hallo Welt.

Actual behavior

babel-german is installed automatically, then the render aborts:

! Package babel Error: You haven't defined the language 'ngerman' yet.
(babel)                Perhaps you misspelled it or your installation
(babel)                is not complete.

babel-german checks \iflanguage{ngerman} at \begin{document}, and without the hyphenation patterns of hyphen-german babel treats the language as undefined. Missing hyphenation patterns are only handled by Quarto when babel reports them as a warning on a successful compile, so this fatal error finds nothing to install.

I can reproduce on current main (a57cc76). After the failed render, tinytex::tlmgr_install("hyphen-german") makes the same document render.

Expected behavior

Quarto installs hyphen-german and the render succeeds, as it does for other languages.

We could detect this babel error on the failure path and install the matching hyphen-<lang> package.

Related: #12732, #10291