Вы можете использовать Array.reduce
следующим образом:
const source = [{"englishText":"dog","languageText":"hund"}, {"englishText":"cat","languageText":"katt"}, {"englishText":"house","languageText":"hus"}]
const target = source.reduce((obj, o) => {
obj[o.englishText] = o.languageText;
return obj;
}, {});
console.log(target);