"sys_expe_attack_1_3_3"

This issue https://github.com/steemnova/steemnova/issues/12
Languages other than Polish lack of this message string.

Btw. shouldn't it be that when I send 1 probe with the fleet on expedition I receive a raport about frequency of expeditions in that particular solar system?

Indeed, vanilla ogame have this type of report. SteemNova doesn't count frequency of expeditions in each solar system. We don't know the exact formula :/