Translation:What kind of animals need salt water in order to live?
While technically correct, this sentence is ambiguous and poorly written. I would rather say: "Hva slags dyr er det som trenger saltvann for å leve?"
Compare: "Hva slags drivstoff trenger bilen for å kjøre?" (What kind of fuel does the car need in order to run/drive?), where 'drivstoff' (the requirement) comes before 'bilen' (the requiring entity).
Either is fine. I think the g is more prominent with older people.
The Norwegian language has been heavily influenced by Danish from the time of Denmark-Norway. Soft consonants are one of the most apparent indications of that, but they are being phased out of the language. For example, it used to be "håber" (håper), "måde" (måte) and "lige" (like). I believe that our tendency to say "shlaks" is a natural (non-prescribed) adoption of that transition -- or maybe just an artifact of pronunciation.
Interestingly, most people I know say "I like måte", but write "I lige måde", even though that spelling is is not valid any more.
(Note that "slag" (kind) and "slak" (soft, loose -- en slak bakke) are different words, and "hva slaks" would be incorrect spelling. For now.)