Hi David,
I think that’s not normal. 
Maybe there is a locale bug. My best tip is the set locale from accept language header setting.
As I see you using this on your site and maybe that’s why you see the default texts on modal because your browser language different and Discourse return it to the default text.
Can you turn off this setting and try again?
Now I can replicate this. I changed the Chrome language from English (United Kingdom) to English (United States) and this happened.
and go back to English (United Kingdom)
