Previous research has established that naturally produced English clear speech is more intelligible than English conversational speech. The major goal of this paper was to establish the presence of the clear speech effect in production and perception of a language other than English, namely Croatian. A systematic investigation of the conversational-to-clear speech transformations across languages with different phonological properties (e.g., large versus small vowel inventory) can provide a window into the interaction of general auditory-perceptual and phonological, structural factors that contribute to the high intelligibility of clear speech. The results of this study showed that naturally produced clear speech is a distinct, listener-oriented, intelligibility-enhancing mode of speech production in both languages. Furthermore, the acoustic-phonetic features of the conversational-to-clear speech transformation revealed cross-language similarities in clear speech production strategies. In both languages, talkers exhibited a decrease in speaking rate and an increase in pitch range, as well as an expansion of the vowel space. Notably, the findings of this study showed equivalent vowel space expansion in English and Croatian clear speech, despite the difference in vowel inventory size across the two languages, suggesting that the extent of vowel contrast enhancement in hyperarticulated clear speech is independent of vowel inventory size.