I think the reason is the set expression "زبانِ شیرینِ فارسی". I'm not really sure how it came to be, but the language has long been described as "sugar" in Persian literature. For instance, this famous verse by Hafez: شکرشکن شوند همه طوطیانِ هند زین قندِ پارسی که به بنگاله می‌رود Or the title of the first Iranian short story "فارسی شکر است", by Mohammad-Ali Jamalzadeh. After all, there are few things that are in touch with your tongue more than a language, and sweet is apparently how Persian tastes, at least to "فارسی‌زبانان" (native Persian speakers). ;)

Oh you know, the word ''شیرین'' has a few meanings in Farsi. We can't really say the English word ''sweet'' is an equivalent Because one of the meanings can be translated as sweet & not the other two. Please take a look at these examples: ریاضی درس شیرینی است Mathematics is an enjoyable subject. زبان شیرین فارسی the enjoyable/lovely Persian language این شربت خیلی شیرینه؛ من نمیتونم اینو بخورم This beverage is very sweet, I can't drink it. همکار جدید کیه؟!!! خیلی شیرین میزنه Who is the new colleague? He's such a slow-witted idiot. Easy?!! That's it, Good Job! Now you know it better than me!!!

زبان فارسی شیرین است=The English language is sweet درس ریاضی شیرین است یا شیرین نیست=The math is sweet or is not sweet "I am a native speaker of English"=من یک انگلیسی زبان هستم=I am an English native speaker"

همانطور که شیرن برای توصیف یک شخصی در انگلیسی به کار می رود در فارسی هم اگر برای بچه ها به کار رود معنیش تقریبا مشابه زبان انگلیسی هست مثال: بچه ها خیلی شیرین هستند معنی: بچه دوست داشتنی و خواستنی هستند

sweet is somehow equal to pleasant and lovely both in meanings of the words and expressions and the rythm. BTW love your profile picture ahah