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Educational evaluations are often conducted with the aim of international comparison. Tests in bilingual or multilingual contexts face the challenge of controlling most of the potential external influences on the domain of the assessed subject. Assessments made one populations with a mother tongue other than the language of evaluation, can present variations in performance because of linguistic characteristics. The effect of the indigenous language as a mother tongue in the scores obtained by students in educational tests in Spanish language is assessed, in the Third Regional Comparative and Explanatory Study of UNESCO in 2013. In a sample of 6th grade students from Educational Institutions of Paraguay (N = 3,015) who answered a written tests, the conducted analysis corresponds to the decomposition of the variance of observed scores taking into account cultural, social and linguistic factors. It was observed that 69% (n = 1809) of the population of the analysis use at home a different language than the one used in the tests and that this is one of the mayor influencing factors on the variation of the observed scores. The use of research recommendations for building assessment tests in Castilian / Guarani bilingual context is proposed.