A given character may render differently in various locales, and fonts on the
host system may be tagged with locale metadata in order to support this.
If the text renderer needs to find a fallback font for a character, it should
pass the preferred locale to Skia's font manager which can use it to find the
best matching font.

The font collection also needs to maintain different caches of fallback fonts
for each locale.

See https://github.com/flutter/flutter/pull/17879