YOU SAID:
My friends are so stupid
INTO JAPANESE
私の友達はとても愚かです
BACK INTO ENGLISH
My friends are very stupid.
INTO JAPANESE
私の友達はとても馬鹿だ。
BACK INTO ENGLISH
My friend is very stupid.
INTO JAPANESE
私の友人はとても馬鹿だ。
BACK INTO ENGLISH
My friend is very stupid.
That didn't even make that much sense in English.