If the sender of a thank has set an other language than the default on the wiki, and then sends a thank (or other notification), it will arrive in that language even if the recipient has another language. I guess this is due to how the logging subsystem works.
Now, if the sender changes language back to the default, then the notifications (thanks) still arrives in the previous language.
I did some testing changing from the default Norwegian (bokmål) at nowiki, and to English, and sent thanks to user Profoss, and they arrived as English text fragments. Later on changing back to Norwegian (bokmål), sending a few new ones, and they to arrived in English.
Now, when thank-notes arrive, I have a mix of Norwegian and English text fragments. In the following screendump note that the first and fifth notification is in English.
It seems like MediaWiki:Notification-header-edit-thank and MediaWiki:Notification-header-flow-thank isn't localized properly, but ext-echo and ext-thanks says everything is localized. Inspecting the actual messages indicates they are translated, that is 18. nov. 2015 kl. 02:46 (history).