As self-initiated expatriates (SIEs) often struggle to communicate in the common corporate language and/or host country language, the aim of this study is to determine if host country language proficiency (HCLP) facilitates communication. As a corollary HCLP might reduce the negative impact of psychological contract (PC) breach on PC violation, and ultimately the positive impact on counterproductive work behaviors (CWB) and the negative impact on organizational citizenship behaviors (OCB). This study uses a repeated-measurement design with three measurement points from 300 SIEs. Using a moderated mediation path model, we found that PC breach at T1 is positively associated with CWB and negatively associated with OCB at T3 through PC violation at T2. Furthermore, HCLP at T1 moderates the positive PC breach–PC violation relationship. Specifically, HCLP amplifies the indirect relationships between PC breach, CWB, and OCB for participants with poor levels of HCLP. This study demonstrates the need of improving HCLP of SIEs. We discuss additional implications for theory, practice and future research.