Natural language is known to play a crucial and specific role for children's learning in school mathematics. Not only does it carry special vocabulary, but subtle differences between natural languages may lead to surprising challenges, for instance, for learners who are not taught mathematics in their mother tongue. In this paper, the anthropological theory of didactic (ATD) is used as a main framework, and we analyse some praxeological anomalies from the teaching of fractions at Japanese schools abroad, while learners attend, at the same time, a regular school in some other language (in this case, the local language, Swedish). Our findings indicate that these praxeological anomalies arise not only from linguistic disparities related to specialised vocabulary and syntax for elementary mathematics but also from institutional and curricular differences. This study gives new insights on these language challenges related to mathematics as taught at expatriate schools, particularly in the case of Japanese.