Our guiding principle is the same as the one for Name Fields: "use their conventions instead of ours."
Applied to locations, this means using place names in native languages and using the names that people at the time used, even if they now no longer exist.
Place names, and even boundaries, change over time. They also have different names in different languages. We aim to use the name that was used by the people in that place, at the time of the event you're recording. This standard is often difficult or even impossible to apply, but it is an ideal that members from all over the world can agree upon.
For example, when recording the birth place of someone born in Port Royal, Acadia, in the 1600s, you should use "Port-Royal, Acadie" rather than the English "Port Royal, Acadia" or the present day "Port Royal, Nova Scotia, Canada".
Use the full place name for counties, states, provinces, départements, etc. Examples: Rhode Island, not RI; New Brunswick, not NB; Hampshire, not Hants; Seine-et-Marne, not S-M. Abbreviation of country names is acceptable as long as the abbreviation is standard and is recognizable.
Tip: If you're unsure of the name in the native language, look it up on Wikipedia. Every place page will say in the first line what the name is in each of the official languages of that country