BERLIN, Jan 25 (Reuters) - Relegation-threatened Hoffenheim have signed Poland international defensive midfielder Eugen Polanski from Mainz 05 as they bid to shore up the league's worst defence, the Bundesliga club said on Friday. Polanski, 26, has signed a contract until 2015, the club's third acquisition in four days after the arrival of defender David Abraham and Ghana international defensive midfielder Afriyie Acquah earlier this week.