Add Different Variants of English to Spellcheck Languages.

### The problem

Currently, there is an option to set the spellcheck language to your OS's system language: [change-spell-check-language](https://github.com/desktop/desktop/pull/11912).

But when I go to apply this option, it still keeps the spellchecker language set to American English (en-US), despite my system language being `English (Australia)` (en-AU).

IDK if this is just an issue faced by system languages that Election's default spell-checker doesn't have a dictionary for, or if this is also an issue faced by people with UK English (en-GB) set as their system language; but a fix that also considers the (en-AU) system language would be greatly appreciated.

### Release version

3.5.2

### Operating system

Windows 11

### Steps to reproduce the behavior

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### Log files

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### Screenshots

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### Additional context

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