* Serbian: Often considered mutually intelligible with Croatian, though some dialects might be more difficult to understand. The two languages share a common literary standard (Serbo-Croatian) and have very similar grammar and vocabulary.
* Bosnian: Also very similar to Croatian, with high mutual intelligibility. Bosnian and Croatian share a common literary standard, and their differences are primarily in vocabulary and pronunciation.
* Montenegrin: Montenegrin is considered a separate language, but it is still very closely related to Croatian and Serbian. Montenegrin has a somewhat different grammar and vocabulary, but it is generally considered mutually intelligible with the other South Slavic languages.
Other South Slavic languages, such as Slovene, Macedonian, and Bulgarian, are also related to Croatian, but they are not as closely related as the languages mentioned above.
While Croatian speakers will have a relatively easy time understanding Serbian, Bosnian, and Montenegrin, there might be some regional variations that influence comprehension.