Allow selecting a specific language for a given email's reply headers

[onceuponaban]

Similarly to #179, I would like to suggest the ability to select a given language among the supported locales as you're about to reply to an email, overriding the addon's default locale setting, either as a drop down menu from the reply button, or RWH's own button depending on which is more feasible. This would be useful for people who write emails in different languages from the same email address.

As the resulting drop down menu would be rather large if it included all the available locales which might be inconvenient, I would also suggest to put a way to select which languages will be shown to the user as part of the addon's settings. The default setting should have none selected initially, effectively disabling the feature so it can be enabled on a per-user basis according to their needs while not cluttering up the UI for those who have no need for this.

[jeevatkm]

@onceuponaban Thanks for reaching out.
First, I'd like to make sure I understand. Your request to add a dropdown locale menu on the compose window would allow you to change the RWH header composition to the selected locale during a composition. Is this correct?
If yes, compared to #179, it would be challenging and complex to implement, given the latest TB internal changes on the WebExtension model.
I will give it a try. However, I'm unable to commit.