The country that English-speaking people refer to as Croatia, is Hrvatska. Its citizens are Hrvati: a man is a Hrvat, and a woman a Hrvatica. It's language is hrvatski (jezik).
Actually, the name of the language (hrvatski) is an adjective meaning 'Croatian', and is used (with gender modification) to describe anything else Croatian: hrvatska hrana (Croatian food), hrvatske planine (Croatian mountains), and hrvatska jezera (Croatian lakes).
The Croatian name for Australia is Australija. Its citizens are called (in Croatian) Australci: a man is an Australac, and a woman an Australka. The adjective 'Australian' is australski.
You may know from a previous post (Country names - noun or adjective?) that the names of some countries are nouns (eg. Australija, Tajland, Kina) and the names of others behave like adjectives. (eg, Hrvatska, Franscuska, Mađarska).
It can be useful to know these terms for any country that you may be talking about, so I have compiled a reference database below - for my benefit as well as yours. The countries are in alphabetical order of their English names.
Bonus: Also included, in the last column, are the appropriate adjectives, such as the words for Italian, Spanish, and American.
You will notice (in fact, you probably already know) that in Croatian, adjectives are not given a capital letter - even when referring to national characteristics. So, for example, the English language (with capital E) in Croatian is engleski jezik (with lower case e).
For many countries, the adjective is the name of the language - such as talijanski, hrvatski. But this is not the case for India, for example, because there are many languages in India, none of which are called Indian.
And although the adjective that refers to things pertaining to Australia is australski, this is not the name of its language because the official language of Australia is English.
There are some gaps in the following list, either because (i) I can't find what we call the residents of some countries, or (ii) because it seems that there may not be an agreed term for the residents.
For example, there is not just one word used for residents of Bosna i Hercegovina. In such a case, I think that Croatians simply use the terminology stanovici Bosne i Hercegovine, although sometimes colloquially, bosanci.
D, E, F
I, J, K
N, O, P, Q
T, U, V, W, X, Y, Z