Ваш вопрос непонятен, но я предполагаю, что вам нужен ключ, значение которого является объектом и который содержит ключ «язык», и этот ключ «язык» должен иметь значение «BENGALI».
//In the case of multiple keys
var obj = {"bn_IN":{"status":"SUCCESS","language":"BENGALI","message":"Translated successfully"},"sq_AL":{"status":"SUCCESS","language":"ALBANIAN","message":"Translated successfully"},"af_ZA":{"status":"SUCCESS","language":"AFRIKAANS","message":"Translated successfully"},"da_DK":
{"status":"SUCCESS","language":"DENMARK_DANISH","message":"Translated successfully"},
"ar_DZ":{"status":"SUCCESS","language":"ARABIC","message":"Translated successfully"}}
var keys=[]
for(key in obj){
if(obj[key].language==="BENGALI") keys.push(key)
}
// In case if you know that key is unique
var obj = {"bn_IN":
{"status":"SUCCESS","language":"BENGALI","message":"Translated successfully"},"sq_AL":
{"status":"SUCCESS","language":"ALBANIAN","message":"Translated successfully"},"af_ZA":
{"status":"SUCCESS","language":"AFRIKAANS","message":"Translated successfully"},"da_DK":
{"status":"SUCCESS","language":"DENMARK_DANISH","message":"Translated successfully"},
"ar_DZ":{"status":"SUCCESS","language":"ARABIC","message":"Translated successfully"}}
var key="";
for(key in obj){if(obj[key].language==="BENGALI") key=key}