I've tested version 7 on bender theme and its same result. After you subscribe to alert... not really sure how or why the language changes to english. Is it possible the system to avoid the original .po files and read directly from the source?
However i've tested your 3th version and it seem to be working fine and not mixing my language but...
1. There is no sorting by relevance
2. When unregistered user subscribe to search and the validation link is clicked from the email there is an error message "There was an error. Please contact us". Further investigation reveals that the subscription went fine but the alert is inactive and the admin have to activate it manually. (not important at the moment)
My question is - Can you combine 3th version to search by relevance too?
Im really eager to provide my users with relevant results of their searches.