It’s croatian. The same layout is used in all countries that used to be Yugoslavia, meaning Slovenia, Bosnia and Herzegovina, Serbia, Montenegro, Macedonia all use pretty much the same layout. Serbian and Macedonian might also have aternative cyrillic layouts, not sure about that.
Those 5 keys are all that are needed. Alt-gr is still used, though to access the symbols displaced by the extra glyphs, so we have stuff liike @[]{} on the third alt-gr layer.
It’s almost the same layout as ISO-DE (so also QWERTZ), except you have ČĆĐŠŽ instead of the german umlaut letters. And for some reason the brackets etc. on the third layer are in completely different places, even though it’s the same characters.
I had the idea of designing a Slavic keycap kit similar to the usual NorDe kit, to cover most slavic languages including Czech, Polish, Slovak, Bulgarian etc… then I went looking at the layouts used in all the various countries and it’s a complete mess 
Even though most of these languages use a lot of similar glyphs, the layouts are all over the place, it would be mission impossible to cover most of them with a keycap set of a reasonable size