You cannot cross-install. A USA copy of Pokémon X requires the USA Update 1.5. An EUR copy requires the EUR update. Attempting to install the wrong region via FBI or BigBlueMenu will either fail or corrupt your game data.
Why is this version so important? While the official eShop served encrypted versions locked to a specific console (3DS), the version allows users to mod the game, play on emulators (like Citra), or install the patch manually on Custom Firmware (CFW) without connecting to Nintendo’s now-defunct eShop servers for the 3DS. Pokemon X- Update 1.5 -Decrypted- 3DS -EUR USA-...
Always dump your own games and updates from your personal 3DS when possible. Only download decrypted files if you own the original cartridge or digital license. Have questions about installing Update 1.5 on your specific CFW setup? Leave a comment below (or consult the r/3DShacks subreddit for updated tools). You cannot cross-install