[Mac] Can we do language tagging?

Ideally, we want to list each keyboard separately through the Apple IM UX, rather than the approach we currently take of having Keyman listed as an IM with each keyboard in a submenu under that.

@jahorton wrote:
>For iOS: https://developer.apple.com/documentation/uikit/uiinputviewcontroller/1618200-primarylanguage
>
>If Apple's done it for iOS, there's likely something similar for macOS input methods somewhere. Linking it here since the paradigms involved are likely similar.
>
>... but, I'm having a lot of trouble finding anything similar on the macOS end.

Tom wrote:

>An IM can definitely be installed as a keyboard for a specific language, as long as it's one of the ones that is predefined in the OS. However, I think it would require a completely separate installation.

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Per https://docs.google.com/document/d/1nQi65m2i9DrAmwG_ahdYtn5JcWYTwI_kJoSLeK2nPAs/edit:

It appears that the list of native languages is fairly limited and not extensible, so there is no way to identify a new language (either in Language & Region, or in Input Methods) by entering a new BCP-47 language tag. Also, I have not yet been able to find any way to install/develop a new input method for an existing language. Keyman for macOS is added as a keyboard under “Multiple Languages”. And it looks like maybe that is the only way. (More research needed…)

[This page](http://belkadan.com/blog/2012/04/Keyboard-Adventures/) notes that TISIntendedLanguage is a Plist key that can be used to supply a BCP-47 value to indicate the intended language for a keyboard. Keyman specifies “mul”, which causes the UI to show Multiple Languages.

"However, iOS and OS X only support the language ID syntax consisting of a language designator and optional region or script designator." - https://developer.apple.com/library/content/documentation/MacOSX/Conceptual/BPInternational/LanguageandLocaleIDs/LanguageandLocaleIDs.html

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See also:
* #170
* #1401