Document that text.lang is case-insensitive, or make it lowercase-only #6549
Description
Description
I'm somewhat used to the codes like ru_RU, so I was surprised to see that these things are case-insensitive in Typst. It reduces user error, but I feel a short sentence or a few words should be added.
typst/crates/typst-library/src/text/lang.rs
Line 132 in 87c5686
Same for region and script.