For what it's worth, mainstream OSes will also have poor handling of these cases, which will eliminate the most tricky cases by the sheer inconvenience it cause

For what it's worth, mainstream OSes will also have poor handling of these cases, which will eliminate the most tricky cases by the sheer inconvenience it causes. As far as I know textfields only have one font applied, so entering both languages in a single field won't be optimal. And if you're not doing anything fancy with your fields, they will all take the same font as well. So even at the input level, the user switching languages will already be mildly screwed, and the best solution would probably be to change pages for each language.