I understand how they translate nouns, adjectives etc but how on Earth do they know that Ramesses was pronounced Ramesses when ancient Egyptian died out thousands of years ago.
Historians don't know how to pronounce proper names. It isn't their domain. If there no accepted orthography (such as "Ramesses") they might borrow the one used by people specialized on the language in question (such as "Nfr m3ˁ.t").
The latter reconstruct them based on how they are recorded in neighboring languages (Caesar is written as "Kaisar" in contemporary Greek and as "QYSR" in contemporary Hebrew), how they are spoken in living descendant languages (Sesar in French, Tshezare in Italian, Tsezar in Romanian, Kesar in Sardinian) and how they are spoken in unrelated languages which have borrowed that name (Kaiser in German, Kesar in Old Church Slavonic and Caesar in English). Sounds don't change at random. Where they can possibly drift to through X generations is the domain of historical linguistics. For Egyptologists such approach is still possible. For Classical Chinese or Elamite it's getting much more difficult.