The BlueSnap Extended Payment API uses language codes as defined by ISO 639-1. In your resource, the language code value should be put inside a language property.
These are the supported languages:

CodeLanguage
arArabic
csCzech
bgBulgarian
daDanish
deGerman
enEnglish
esSpanish
fiFinnish
frFrench
heHebrew
huHungarian
itItalian
jaJapanese
krKorean
ltLithuanian
nlDutch
noNorwegian
plPolish
ptPortuguese (Brazilian) / Portuguese (European)
ruRussian
skSlovak
slSlovenian
sqAlbanian
srSerbian
svSwedish
trTurkish
zhChinese (Simplified) / Chinese (Traditional)