OK, fair enough
So here’s some clarification for those who want to read it :
First of all, having a “Title” and an “Original Title” field is not ideal for 100% of the cases. Sure, it works for most movies, so most people are happy in most cases. At least for people living in english-speaking countries. Now let’s take three examples.
EXAMPLE 1 :
The well-known movie “Spirited Away” from Ghibli Studio has an original title that goes like this : 千と千尋の神隠し, Sen to Chihiro no kamikakushi. Not being a japanese reader, I couldn’t care less about such a title. And the same could be said about all asian movies (e.g. the famous “Seven Samurai”).
However, for such movies, instead of the original title, I would appreciate to have the ENGLISH and the FRENCH titles, since France happens to be the country where I live (yup, not everybody lives in english-speaking countries, you know
). So in the case of “Spirited Away”, I would appreciate to have the french title, which is “Le voyage de Chihiro” (Chihiro’s Journey).
EXAMPLE 2 :
The also well-known french movie “Amélie” has an original title that’s much more poetic : “Le fabuleux destin d’Amélie Poulain” (Amélie Poulain’s Fabulous Destiny). I for sure would like to keep that title. And since I live in France, I would even like to make that title my main title (the one used for displaying / sorting), instead of the english one.
EXAMPLE 3 :
I’m a spanish guy. I come from Spain, I just happen to live in France. I speak spanish perfectly. Now take any spanish movie, like Almodovar’s “The Skin I Live In”. I for sure would prefer the original spanish title “La piel que habito”.
This is why having a “Title” and an “Original Title” field is not ideal. A “Main Title” and “Alternate TItle” fields would be much better. Even better if we could choose “Language for Main TItle” and “Language for Alternate Title”, on a movie per movie basis.
That way, I would be really happy because :
- For all american/british movies, I could have “Main Title” = english and “Alternate Title” = french
- For all french movies, I could have “Main Title” = french and “Alternate Title” = english
- For all spanish movies, I could have “Main Title” = spanish and “Alternate Title” = whatever
- For all asian movies, I could have “Main Title” = french and “Alternate Title” = whatever
And of course I could choose to sort my movies, either by “Main Title” or by “Alternate Title”.
Do you see now how much better this could be for people like me ? 
But since Plex only provides a “Title” and an “Original Title” at the moment, my purpose was/is to use the “Title” field as “Main Title” and the “Original Title” field as “Alternate Title” (by entering all the relevant data then permanently LOCKING the fields), hoping that one day Plex developers can take care of this and offer “Main Title” and “Alternate Title” fields instead (which would make 100% of the users happy with no downsides IMHO).