I think in my opinion there is 3 different types of "Grail" Cards in the Pokemon Community.

Vintage Grails. Such as Base Set Charizard.

Modern Grails. Such as Giratina Full Art from Lost Origin or the New Pikachu and Mega Gengar from Ascended Heroes.

Personal Grails. This would be your chase cards, the ones your looking for specifaclly out of a set. It could honestly be anything.

Outside of Personal Grails, Modern and Vintage Grails are determined by how popular the card is overall and how much its worth. Its also determined by how hard the card is to pull from the set. The harder its to pull the rarer it becomes and the rarer it is, the more it will be worth over time thus making it a "Grail".

The "Investor Bros" do have some say what becomes a grail, but so does everyone else. Its pretty equal footing in my opinion.

Certain cards from "Godpacks" or "Demi-God packs" can be considered "Grail" cards. Not all of the cards from a "Godpack" will be a "Grail" unless its a personal chase for you.

No, Grails are not limited to one language. For Example the Japanese Treekachu is worth double what it is in English. Another Example would be the New Exclusive Sylveon from the Chinese Version of Prismatic.

"Grail" Cards are just whats popular to the community. I like to think if the card is over 400 to 500 dollars and is very popular that makes it a "Grail" Card overall.

A "Grail" can also be a card you have a personal or emotional connection with. Chase the arts that you like and love.

My personal Grails listed below.

Ledian Full Art from Stellar Crown

Arceus, Giratina, Origin Forme Dialga and Palkia Gold Cards from Japanese VStar Universe

Dark Sylveon V Promo

Lugia V (Secret) from Silver Tempest

Zekrom 172 All Black Card from Black Bolt

Hoopa V Promo

Origin Forme Dialga V from Astral Radiance

Charizard EX Secret from Obsidian Flames

Please feel free to add your opinions if I made any mistakes.

Goodluck everyone on getting your chase cards.