The issue with trying öäå on your keyboard is that the font used then does not include the proper glyphs.
But you will only need to type characters from the
ASCII set during installation, so do not type any non-ASCII character when trying the keyboard after having installed the keymap sv-latin1.
I think that you can then proceed to Slackware installation without issue.
Incidentally I have checked that in Slint (Slackware derivative, see in my signature), for Swedish we do set sv-latin1 by default and this has probably been tried at least by our Swedish translator