Grapheme-color synesthetes experience graphemes as having a consistent color (e.g., “N is turquoise”). Synesthetes’ specific associations (which letter is which color) are influenced by linguistic properties (letter frequency, phonetic similarity, etc.), and plausibly reveal the characteristics of underlying letter representations. Despite their clear linguistic origin, these influences (termed “Regulatory Factors”, RFs) are almost always studied in a single language, typically English. Here, we measure the influence of three RFs (previously examined only in English) in Dutch, English, Greek, Japanese, Korean, Russian, and Spanish synesthetes. For two RFs, effect size significantly differed between languages. In contrast, an RF which we predicted to be universal (because it is shaped by prelinguistic shape-color associations) did not differ between languages. Our results suggest that monolingual synesthesia studies should be interpreted with caution. Furthermore, they show how synesthetic associations offer an exceptional opportunity to study linguistic and prelinguistic influences on letter representations in different languages.