Tomado de la wiki oficial de LumaLocale Emulation: While Luma3DS allows games from all regions to be played via region-free patches, some games will freeze or crash if outside their expected region/locale. To fix this, Luma3DS allows emulating your locale.
To use this feature, create a locale.txt inside /luma/titles/TID/ file. Edit this text file to include the region (JPN, USA, EUR, AUS, CHN, KOR, TWN) and language (JP, EN, FR, DE, IT, ES, ZH, KO, NL, PT, RU, TW) separated by space. For example, to play Pokémon Y in the Japanese language and region, I would place a locale.txt in /luma/titles/0004000000055E00 containing the contents JPN JP
Note: This level of locale emulation is not sufficient for some games; to fix this, improved locale emulation was introduced in Luma3DS v8.0, allowing you to specify a country and state ID (2-digit hex integer), i.e. a European locale.txt played in French would have the contents EUR FR FR 01. This extended locale emulation is not necessary in most cases.