Added slovak language file
I'm also translating Riot.im and Riot Android at https://translate.riot.im/ so if nothing else my translation is at least in sync with other similar apps.
As for Fractal all the translatable strings are now translated and I have loaded my language file in my local build for a short testing.
In the past I've also contributed a few translations to GNOME apps such as gnome-sound-recorder, gnome logs, polary and orca.