"This French expression is very funny."

Translation: Cette expression française est très drôle.

[Nathan173901]

Laquelle?

[GrahamNdP]

There are many, but top of my list would be: « Il pleut comme vache qui pisse. » ! :p

[MarilynMil14]

why isn't amusante accepted, in place of drole?

[GuyPrend]

Is "cette phrase française" acceptable for "this French expression"?

[choracavaco]

Not really: "cette phrase" = 'this sentence'.