Collection about short stories including very short ones

I first apologize for the lack of details: I read this collection nearly 30 years ago.

I remember only a very few details about it:

• Stories were of various size but at least include a few very short ones
• They were mainly (only?) SF stories
• They were "smart" stories, whose plot was usually the reason why the story was short.
• I remember one story that does exactly match the previous fact (I think it was only 1 or 2 sentences short) : Aliens arrived near earth, saw a sign that said "polluted planet", and decided to fly back home (sounds like that)
• I read it in a French version. The cover might have been light blue (but I am unsure)

This may match a lot of collections of SF short stories, but I hope the only one I remember will help find the one I am looking for!