Enable spell check

[chrisbobbe]

A user reports that spell check isn't working: #mobile > spell check on iOS @ 💬

We found that spellcheck is off by default in Flutter apps but can be enabled with a TextField param.

When I try that on iOS, though, I still don't get spellcheck feedback, because of issues in how the app decides which language to use:

#mobile > spell check on iOS @ 💬

Multiple unexpected things:

• It seems like "en" isn't in UITextChecker.availableLanguages, even though we've decided that's a valid locale to show the app in
• UITextChecker.availableLanguages isn't being checked properly:
  • We should be querying it for "ISO 639-1 language codes or combined ISO 639-1 language codes and ISO 3166-1 regional codes (for example, en_US)".
  • Instead, we'll end up querying it for things like "zh_Hans_CN" which isn't [that].

Upstream issues

• SpellCheckConfiguration for TextField doesn't work with AppLocalizations on iOS flutter/flutter#151713
• Spell checking should be according to current selected language in keyboard, not locale of device. flutter/flutter#138139

[chrisbobbe]

Quoting Greg from that discussion:

One thing to decide is which text fields we'd want spell-checking enabled on.

• Definitely the main compose input.
• Probably the topic input?
• Probably not in the new-DM search field where you're typing someone's name, though — spell check seems more likely to be annoying than helpful there.
• Similarly probably not in the login flow where you're typing your server URL, your email, or your password.
• Not sure about the search query input when you're searching for messages.
• Not sure if there are other places in the app where spell check would be helpful.