"Wife-to-be" refers to the woman the person in question is going to marry. You can also say fiancée, future wife, prospective wife. Some people refer to their partners as their spouse, although they aren't legally married yet, but I would use "fiancée".

You could use fiancée instead. Wife-to-be means the woman he is going to marry. If they're not yet married, you wouldn't call her his wife yet.

The 'to-be' is a marker of something in the future that I am progressing towards but the outcome is not certain ... I want to be an astronaut but I don't know if I can be; I'm an astronaut-to-be... Wife-to-be ... Might be my wife but until we are married it is uncertain... You are an English speaker-to-be :)