I'm not sure I completely understand what you're seeing, but in any event it depends on the desktop environment you chose when you downloaded the Solus .ISO file. Most of my experience is with Budgie and Plasma. It sounds like your localization process isn't begun until after you've typed your password. Is that right? That may be intentional to allow different users to have the language they need. (I've only used English, so that's a guess on my part.)
That said, in the Plasma DE, there's a small keyboard icon in your lower left corner on the logon page, that displays a virtual keyboard on the monitor. You can type on it using the mouse. When I display that keyboard, the words American English appear at the bottom. That's my keyboard choice, so it appears that the virtual keyboard is localized. Thus, the Plasma DE might be the solution you need.
Budgie doesn't provide that virtual keyboard. I'm not sure whether MATE or GNOME does, as I haven't used those in a long time, and don't remember whether they offer a virtual keyboard. One thing is for sure, though. This forum has enough participants that localize to something other than English, so I'm pretty sure you'll get a better answer soon. Enjoy Solus!
