Conclusion / Solution
Ultimately I'll be going with using Firefox as it's support seems to be more thorough.
Here is my solution:
var fp = new FirefoxProfile();
fp.SetPreference("intl.accept_languages", "en-au");
desiredCap.SetCapability(FirefoxDriver.ProfileCapabilityName, fp.ToBase64String());
For Chrome, it's support of language is limited to ISO 639-1, meaning it only supports 2 character languages. I attempted for en-au, en-ca, etc and it seems to just add English (UK) in these cases.
Here is a summary of the options I tried and their results:
var options = new ChromeOptions();
//options.AddArgument("--lang=es"); //espanol
//options.AddArgument("--lang=es-mx"); //espanol (Latinoamerica), espanol
//options.AddArgument("--lang=en-ca"); //english (UK), english (us), english
//options.AddArgument("--lang=en-au"); //english (UK), english (us), english
//options.AddArgument("--lang=en-nz"); //english (UK), english (us), english
//options.AddArgument("--lang=zh"); //english (us), english
//options.AddArgument("--lang=zh-tw"); //Chinese (Traditional Chinese), Chinese, english (us), english
//options.AddArgument("--lang=zh-hk"); //Chinese (Traditional Chinese), Chinese, english (us), english
//options.AddArgument("--lang=zh-cn"); //Chinese (Simplified Chinese), Chinese, english (us), english
options.AddArgument("--lang=fr"); //Francais (France), Francais, english (us), english
//options.AddArgument("--lang=fr-ca"); //Francais (France), Francais, english (us), english
//options.AddArgument("--lang=aus"); //Francais (France), Francais, english (us), english
desiredCap.SetCapability(ChromeOptions.Capability, options);
a
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