Would someone in German call someone silly in a friendly, playful, teasing way? (like couples or friends would do) — e.g. "you're so silly"
I found "albern" and "Dummerchen". Would you be offended if someone close call you that in a playful way?
I would be surprised, not offended. Those words are not used in this way. "Albern" is definitely not usable in a funny way, it is purely negative. "Dummerchen" (or "Dummchen") is not particularly much in use, and not that funny either.
Also Google translator says that "Dummerchen" means sausage...?
Now that is silly.
If you wouldn't exactly call someone silly in German and sound cute, is there any "insult" that is cute, innocent and used just for the purpose of teasing?
It's hard. I'm native German, and I always thought "silly" had rather harmless connotations (i.e., usable in that funny way you intend). But if I translate it at, e.g., dict.leo.org, then I get many german translations which are by all means not harmless. Maybe my understanding of the word "silly" is not up to date.
Thinking more about it, I can't really find a strong single candidate that means "silly but harmless / funny / still lovable" or something like that.
Maybe "niedlich", depending on context and intonation. Also "süß" (especially when used male-to-male in a non-sexual context).