Pre defined "medium" date format for Slovak is wrong - adds 2x extra spaces between day-month and month-year. This extra space causes e.g. failure in PHP ICU when parsing SK locale date. Once spaces removed, there§s no failure - therefore I reckon the root issue is caused by ICU library, not by PHP implementation.
For instance formatting "2019-01-01" results into "1.[space]1.[space]2019" (1. 1. 2019) - those spaces should not be there.
Long format is OK (e.g. 1. októbra 2019 is OK with spaces) as well as short format (e.g. 1.1.19 - without spaces).
This issue might be also applicable at least to Czech locales AFAIK - might need to check.
Tested in ICU 63.1. / PHP 7.2.
From ICU data dump at
I would guess that it's the section "generic -> DateTimePatterns -> item 6" which need to be corrected from current "d. M. y G" to "d.M.y G" (drop 2 spaces). Just my guess.