The goal of this paper is to investigate what determines the viability of alternatives in pragmatic reasoning about scalar implicatures. Data from English and Spanish speakers is compared to investigate whether cross-linguistic differences in scalar implicature calculation are based on how idiomatic an alternative is. The data supports the idea that viability of an utterance as an alternative is determined based on more than simply complexity. Namely, alternative utterances are viable based on how prevalent, or common, they are. Languages differ in regard to which utterances are considered viable because translational equivalents in some languages are more widespread than others in a different language. When comparing two Rational Speech Act models, a prevalence-based model, which considers production data that I collected, significantly outperforms a complexity-based model that considers length of utterance in words. This suggests that listeners are Bayesian -- they are informed by production probabilities, which reflect how widespread an alternative is, when calculating scalar implicatures. Additionally, my findings provide evidence against a structural approach to calculating alternatives (Horn, 2000; Katzir, 2007), favoring theories that determine alternatives based on production probability (Geurts, 2011; Goodman & Stuhlmüller, 2013).