Why are Kakyoin and Polnareff so pleased about knowing the same hand signals?
Polnareff knows this by virtue of him being Polnareff.

Combining the individual gestures and their 'translations' makes the sentence that it does.

Pan (Japanese sound for clapping) + 2 (two) + ok (maru) + seeing something (mieru) = pan-tsu (underwear) + maru (entire) + mieru (can see)

Only Kakyoin is able to decipher what he's trying to say, since NO ONE ELSE chimes in, and they both share a bonding moment. That bonding moment is the amazing handshake, and their eyes darken to reflect the solemnity of the bro-ment.