See also Croatian, Serbian, and Bosnian. I also find Chinese to be interesting, e.g. Mandarin and (formal) Cantonese have a near identical written language, while the spoken language is completely different, views on whether or not those languages are different languages or dialects vary wildly.
In my books, the distinction between languages and dialects are so arbitrary that the best method is simply to ask the people that speak those languages/dialects. If they consider them to be different language (which Maltese speakers seemingly do) I call them different languages.
Mandarin-Cantonese is very interesting and a unique (to my knowledge) example where the same written language can be completely different to two different people.
I don't buy the argument of just asking the speakers. There are cultural, political, etc. reasons people may think things which don't conform with reality. Many Hindi-Urdu speakers get insulted by the reality that the languages are pretty much the same because they don't want to identify with people from another country their country is constantly at war with.
In my books, the distinction between languages and dialects are so arbitrary that the best method is simply to ask the people that speak those languages/dialects. If they consider them to be different language (which Maltese speakers seemingly do) I call them different languages.