Gets the localized string for the specified message. If the message is
missing, this method returns an empty string (''). If the format of the
getMessage() call is wrong - for example, messageName is not a string
or the substitutions array has more than 9 elements - this method
returns undefined.
messageName The name of the message, as specified in the messages.json file.
substitutions Up to 9 substitution strings, if the message requires
any.
returns Message localized for current locale.
Gets the browser UI language of the browser. This is different from
i18n.getAcceptLanguages which returns the preferred user languages.
returns The browser UI language code such as en-US or fr-FR.