“Der Developer” is a loan-word from English that was also formerly mainly used in photography, but is now commonly used for software developers.
English being the current lingua franca of the world, English loan-words are common and fashionable in contemporary German —especially in the spoken language— even where perfectly suitable German words already exist, and even though the German language has a highly productive morphology for coining new words. The influence of English is particularly strong in software, because most programming languages use English keywords and most software concepts are recent and were first expressed in English.
For the purpose of learning German, using native German words in Duolingo exercises will serve you better than using English loan-words. In real translation work, the choice should depend largely on the conservativeness or hipness of the style of the original text.