See, you did not understand my point.
When an user newly registers, this plugin sends our the message automatically as soon as the user registers.
At that point of time, the site will be in only 1 default language. There is no way to determine what language that the newly registered user will want to see the message.
I am with Oxwall for several years. I know very well that Oxwall has a language settings. But this does not related to what you expect.