I fully agree.
At first I thought I'm doing something wrong when I saw that guests received messages in both Bulgarian and English. I was surprised to find out that it wasn't a feature.
So I simply stopped using the Bulgarian messages now. Even Bulgarian guests get the automated English messages.
It's not ideal though, as others pointed out. It's weird for 2 people who speak the same native tongue to send texts in English.
The easiest way for this would be to select the language from the app/browser of the booker and if that language is not included in the list of languages for automated messages, either to be able to select which language is the default for each particular host or if that's too complicated - to have English as the default for all hosts.
It doesn't sound way too hard to be implemented and, if promoted properly by Airbnb to hosts, would be greatly utilized to help guests have a much more personalized experience here on the platform. All the while saving time and effort for the hosts.
Out of which Airbnb will have a great benefit, too.