TIL that although all English Mega Pokémon cards had a Japanese text in the artwork, the Japanese cards only had an English text in their artwork on the Ultra Rare versions. The Secret Rare versions had a Japanese text in the artwork instead. Here for example the four M Charizard EX cards from the Flashfire/Wild Blaze sets:
English Flashfire 13/106 and Japanese M Charizard-EX Mega Battle Deck 002/021 Ultra Rares:
English 69/106 Flashfire and Japanese Wild Blaze 055/080 Ultra Rares:
English Flashfire 107/106 and Japanese Wild Blaze 088/080 Secret Rares:
English Flashfire 108/106 and Japanese Wild Blaze 089/090 Secret Rares:
As you can see, the Japanese Secret Rares have Japanese texts in the artwork as well. Just something I’ve learned just yet and thought was worth sharing. 
Greetz,
Quuador