Regional Card Legality
Magic: The Gathering cards were originally printed in English only. The Legends expansion marked the first internationalized printing of a Magic set, in Italian. Further languages were added over the years until the current set of languages was reached: English, Chinese Simplified, Chinese Traditional, French, German, Italian, Japanese, Korean, Portuguese (mostly for the Brazilian market), Russian, and Spanish.
Yu-Gi-Oh! TCG cards printed in English, French, Italian, German, Spanish and Portuguese can be played in any TCG territory. Cards printed in Japanese, Korean, Chinese or “Asian English” (ie: Yu-Gi-Oh! OCG cards) can only be played in Asian territories and are not legal for use in Yu-Gi-Oh! TCG events.
Pokémon cards are released in English and/or Japanese. German, French, and Italian are pretty common as well for almost all sets, and Spanish (both from Spain and Latin America) for most of the earliest and newest sets (not so much for the EX era). The earlier Portuguese cards were from Portugal; the newer Portuguese cards are from Brazil. Korean usually has some odd sets which are either the same as the Japanese, or a mix of multiple sets, and quite a few promos. Chinese only has two sets: Base and EX Legend Maker; as well as from a Jungle Pikachu from the Pikachu World Collection 2000 set, some POP4 promos and the EX Legend Maker Plusle and Minun Deck. Dutch has Base, Jungle and Fossil; as well as a few WotC promos. Polish only has two sets: Diamond & Pearl and Mysterious Treasures, as well as a Pikachu from the Pikachu World Collection 2010 set. And Russian started releasing sets since XY (although it skipped some, like XY Generations and Evolutions; they will soon release some SM Decks and sets though).
Digimon Card Game cards are printed in two languages: Japanese and English.
Cards printed in English can only be played in some countries or areas ( North America, Latin America, Europe, Oceania ). Cards printed in Japanese can only be played in Asian countries or areas ( Japan, Hongkong, Taiwan, Singapore, Malaysia, Indonesia, Philippines, Thailand ).
Countries or areas which can use English cards・North America・Latin America・Europe・Oceania
Flesh and Blood cards are only printed in English.
FoW TCG cards printed in English, French, German, Italian, Portuguese, or Spanish are legal for play in Sanctioned tournaments within all territories. Cards printed in non-Japanese Asian language are legal for play in sanctioned events everywhere with the exception of European countries. Cards printed in Japanese language are legal for play in sanctioned events everywhere. World Grand Prix, Open and Masters are an exception and all FoW TCG cards are legal regardless their language in these tournaments.
If you live in NA any foreign language cards are legal. EU can't use non-Japanese Asian.