Förster gained the first of his 81 caps for the national team in a friendly with Brazil, on 5 April 1978, not yet aged 20. From then onwards, he became an undisputed starter, appearing in all the matches at the victorious Euro 1980 (completely neutralizing Belgium's Jan Ceulemans in the final) and the runner-up place at the 1982 FIFA World Cup (also playing all the minutes).