I've just downloaded the Lato font family from Font Squirrel and installed it onto Windows 10.
A quick look at the Fonts folder shows me that, for example Lato Bold: Font Embeddability is Installable.
I created a new document using three variants - regular, bold, and light-italic - but Scribus 1.4.6 isn't letting me embed them.
This tells me that there's probably something about those fonts that Scribus doesn't like.
In Scribus, look in "File -> Preferences / Fonts", find the Lato font and scroll to the right. If the "Subset" check-box is checked then Scribus cannot embed the font for one or more reasons given here:
http://forums.scribus.net/index.php/topic,2523.0.htmlUnfortunately there's probably nothing you can do about this. If Scribus won't embed the font then it can't be embedded.
P.S. I have no idea if changing the Subset check-box value has any effect but I'm personally not willing to try in case I cause other problems.