begin{filecontents}{test.bib}
@online{gates,
    author       = {Bill Gates}
    title        = {Save the world!
    url          = {https://www.gatesfoundation.org/de/},
    date         = {2016-07-04},
    organization = {Bill and Melinda Gates Foundation}
}
}
end{filecontents}

documentclass{article}
usepackage[style=authortitle]{biblatex}
addbibresource{test.bib}
begin{document}
Lorem ipsum dolor sit amet, consetetur sadipscing elitr, sed diam nonumy eirmod tempor invidunt ut labore et dolore magna aliquyam erat, sed diam voluptua.footcite{gates}
printbibliography
end{document}

DefineBibliographyStrings{english}{
  urlseen = {visited on}
}