First, a few facts
If you look at the map, you can easily see that there are several other countries on the way to Croatia. The situation in each of them can change dynamically. Some of our editorial staff have just returned from a cruise around Croatia and have traveled the entire route back and forth with their families without any obstacles. Whether this is the norm or maybe they were lucky - it's not for us to decide.
The fact is that regulations regarding the coronavirus can be different. We have no influence on whether, from the moment of writing this text to the moment you read it, some politician will change his mind and introduce new regulations. It may happen. We are not responsible for this - the relevant services in each country are responsible for posting up-to-date information. Therefore, before leaving, check what the regulations look like at the moment on the relevant government websites. Here is a short cheat sheet detailing the countries you can travel through.
Austria
Information for tourists can be found at this address: https://www.austria.info/pl/informacje-ogolne/pozostancie-zdrowi/zasady-wjazdu-do-austria . Up-to-date information on testing, business travel and transit is gathered there. A useful address may also be the official government website: https://www.gov.pl/web/austria/przyjazd-i-tranzyt-przed-Austrie
The czech republic
Our southern neighbors post current information for travelers at this address: https://www.visitczechrepublic.com/pl-PL/926179fc-1601-4cf8-b201-0b33b3878f08/page/covid-19 . It is also worth checking the website and recommendations of our Ministry of Foreign Affairs https://www.gov.pl/web/dyplomacja/czechy .
Slovakia
In this case, the official website of our Ministry of Foreign Affairs remains the best source of information: https://www.gov.pl/web/dyplomacja/slowacja . You can also visit a special website created by the Slovaks in connection with the coronavirus: https://korona.gov.sk/ . There you will find maps, lists of cases, as well as information for travelers.
Slovenia
Current information for travelers is available at: https://www.visitslovenia.pl/artykuly/1,3202,0,0,koronawirus-slowenia . You can also chat there and find out first hand what the situation is like. An alternative source is the website of the Ministry of Foreign Affairs: https://www.gov.pl/web/dyplomacja/slowenia .
Hungary
The conditions for passing through the country of Bratanki can be found at this address: https://www.bud.hu/en/covid_19/information_on_entering_and_leaving_hungary or on the Polish website https://www.gov.pl/web/dyplomacja/wegry .
It is worth remembering that although currently (April 2021) it is possible to drive through Hungary without tests, there is a catch: when entering, you will get a "transit" sticker with which you can only drive on certain roads (you will receive a map together with the sticker) and refuel at designated stations. If you do not comply with this requirement, the police may fine you.
Entry rules to Croatia (as of April 20, 2021)
First of all, remember that Croatia is outside the Schengen area. This means that border control can and should be expected there. What's more, according to Polish regulations, when returning from a non-Schengen country, we should quarantine.
Before leaving, you must also meet one more requirement - register on the website: https://entercroatia.mup.hr/ . Detailed and, above all, up-to-date information on the restrictions in force can be found at this address: https://www.gov.pl/web/chorwacja/wjazd-do-chorwacji-only-z-testem-na-sars-cov2 .
Currently, entry into the territory of Croatia is possible for the following groups of people (before leaving, be sure to check it on the government websites) :
1. Children up to the age of 7.
2. Convalescents (up to 180 days from coronavirus infection).
3. Vaccinated people.
4. Holders of a negative test result translated into English. You can choose for about PLN 130 or PCR for about PLN 400. The test cannot be older than 48 hours ago. You can also do it in Croatia, but it's better to do it in Poland - if it comes out positive and you are far from home, it will be a problem.