Thank you!
I found another way to solve:
ggplot(tabelle, aes(x = Monat)) +
geom_line(aes(y = weiche_Begriffe, color = "1"), linetype = 1, size =2) +
geom_line(aes(y = Extremismus, color = "2"), linetype = 1, size =2) +
geom_line(aes(y = Demagogie, color = "3"), linetype = 1, size =2)+
geom_line(aes(y = Verschwörung, color = "4"), linetype = 1, size =2)+
geom_line(aes(y = Beleidigung, color = "5"), linetype = 1, size =2)+
geom_line(aes(y = Sexismus, color = "6"), linetype = 1, size =2)+
labs(y = "Häufigkeit", title = "Häufigkeiten pro Monat")+
scale_x_continuous(breaks = seq(1,12, by = 1)) ->p
p + scale_colour_discrete(name="Kategorien", breaks=c("1","2","3","4","5","6"),
labels= c("weiche Begriffe","Extremismus","Demagogie",
"Verschwörung","Beleidigung","Sexismus"))