PostgreSQL 9.5 won't start after Windows 10 fall update
Thanks to suggestion from Daniel Vérité I was able to fix it without dumping the entire db server. My understanding of locales on Windows is very limited but from what I've learned it seems like Microsoft changed region name of czech locale from "Czech Republic" to "Czechia" during Fall Creators update (presumably so it conforms to ISO 3166-1). 

Postgres somehow loads locale by combination of language name and region name, so databases created before this change cannot be connected to since the combination of language and region is no longer valid.

So to fix it, I downloaded Locale Builder 2.0 from Microsoft, and then:


Created new locale based on Czech locale (cs-CZ), kept the same locale name (cs-CZ)
Changed region name to "Czech Republic" (from "Czechia")
Created installer by selecting "Build" -> "Build Locale Installer"
Installed the newly created locale by running .msi file created in previous step
Restarted the PostgreSQL server, this time it started without problem


Now I can run PostgreSQL 9.5 with 'Czech_Czech Republic.1250' and PostgreSQL 10 with 'Czech_Czechia.1250' and they both work.

I'm not sure whether it's ok to create the locale with same locale name (cs-CZ), but it doesn't seem to cause any problems, maybe someone can elaborate more.