English-on-Japanese cards are cards that work on the Japanese E-Reader+, but contain all English text.
The E-Reader was much more successful in Japan than anywhere else, and so today the Japanese E-Reader+ is more common and easier to find. E-on-J cards are targeted at those who bought a Japanese E-Reader+ due to it being easier to get, but would prefer English.
How to know if a card is E-on-J
All E-on-J cards contain the "EONJ" logo.
How to scan an E-on-J card
They scan and load just like any other card. But you must use a Japanese E-Reader+. They will not load on a USA E-Reader.
Also make sure you have a Japanese E-Reader+ and not the original Japanese E-Reader. Retro Dot Card games are not compatible with the original Japanese E-Reader. Check out this FAQ answer for more info.
How E-on-J games differ from USA games
E-on-J games are identical to USA games in every way. All games that were released for the USA E-Reader also have an E-on-J version. They contain the same text and same experience. E-Snake does have some abbreviated text.
But, the text on E-on-J games is larger and wider. Here is a comparison of some of the text in all games in Series One.
Exo Attack
Franny Answers
Scavenger Hunt
E-Snake
Solitaire and Bomb Hunter both have no text, and so are completely identical for all regions.