Conclusion / Decision
I will ultimately use Firefox, as support seems more thorough.
Here is my solution:
var fp = new FirefoxProfile();
fp.SetPreference("intl.accept_languages", "en-au");
desiredCap.SetCapability(FirefoxDriver.ProfileCapabilityName, fp.ToBase64String());
Chrome ISO 639-1, , 2 . en-au, en-ca .., , ().
, , :
var options = new ChromeOptions();
options.AddArgument("--lang=fr");
desiredCap.SetCapability(ChromeOptions.Capability, options);