Well, actually I've switched to the Brave Browser and it's loading time is blazing fast. Yes, my SSD sadly is only 128GB, because this is a new notebook and I wasn't able to buy a bigger/better one, yet.
Anyway, thank you for your help, I will leave them alone, because they won't cause any trouble in the future I guess. 
EbonJaeger For Firefox just disable the language packs you don't use from the add-ons.
PS: Old hardware doesn't necessarily mean smaller disks.
Just to chime in because I don't see an actual answer regarding localepurge: we have no such thing since it would mean "breaking" an installed package. eopkg doesn't have a concept of a partial install either.
kyrios For Firefox just disable the language packs you don't use from the add-ons.
I know. One. By. One. That's a LOT of language packs.
EbonJaeger yeah I know, but it's a one shot.
This allows to have a localized firefox out of the box (even on multi user systems on which the users use different locales).
FWIW this is how Fedora handles the language packs
DataDrake Thank you for this information!
Usually, whenever I install a distro that I want to keep it for more than a week (yep, I'm a distrohopper), I use bleachbit. There are some options in it that can remove many unused languages. The only issue in Solus, but a minor one, is that it doesn't start using adm privileges. Just go to terminal:
sudo eopkg it bleachbit
sudo bleachbit
Be happy 
In case anyone else does this, I experimented with using bleachbit as raf_lozano does, to remove locales in Solus and assess the damage. It left plenty broken, but simply running sudo eopkg check | grep Broken | awk '{print $4}' | xargs sudo eopkg it --reinstall fixed everything right up again.
Thanks everybody!
