Issue title: spellCheckHandler.switchLanguage doesn't do anything
 Issue body: I'm adding spellchecker in the webview following the webview example, and currently the language is always being set as 'en-US'. 

The use case here is -  I know what language the user is going to be typing in, so I can set it for them manually, but `spellCheckHandler` seems to be ignoring the string I pass in `window.spellCheckHandler.switchLanguage()`: 

```
window.spellCheckHandler.switchLanguage(formattedLocale); // when eg. formattedLocale = 'pl-PL'
console.log('should set to: ' + formattedLocale); // 'should set to: pl-PL'
console.log('is set to: ' + window.spellCheckHandler.currentSpellcheckerLanguage); //  'is set to: 'en-US'
```

I had a look at the source code, could this be overridden by: https://github.com/electron-userland/electron-spellchecker/blob/master/src/spell-check-handler.js#L149? My system language is not en-US. 

Note that spellcheck works with the en-US locale, including replacing words, etc. I don't see any errors with downloading dictionaries - nothing in the josephgarcia@example.com. 
 Comments: 
 Comment 0: What operating system is this? macOS currently ignores all switchLanguage calls and relies on macOS's garbagetown "Automatic Language Detection"
 Comment 1: @paulcbetts yes, that's on macOS. I figured it's the native language detection after looking through the code. Is there a plan to change that in the future? 

The native detection works well, but it's not documented anywhere in the package, so it was a bit confusing as to what was going on. 
{'source': 'GithubIssues', 'repo_name_with_owner': 'electron-userland/electron-spellchecker', 'issue_id': 213331893, 'repo_stars': 199.0}