bug(ios,mac): Handle localization for languages that are not supported by the OS

> We are going to have to find a way of supporting languages that are not supported by the OS for iOS and macOS. This is going to keep coming up as a problem.

_Originally posted by @mcdurdin in https://github.com/keymanapp/keyman/issues/6047#issuecomment-996425876_

TODO list of language strings already pulled in from crowdin, but not supported by the platform

## iOS
Bura-Pabir (bwr-NG) #5839
Kanuri (kr-NG) #6047
Kibaku (ckl) #6048
Mandara (Wandala) (mfi-NG) #5857
Marghi (mrt-NG) #6049
Shuwa (Latin) (shu-latn-NG) #5734
Waha (hia-NG) #6050

## macOS
Mandara (Wandala) (mfi-NG) #5857
Waha (hia-NG) #6050
Shuwa (Latin) (shu-latn-NG) #6104