This chapter reports on recent mixed method research investigating the comparability between assessment in relation to linguistic and cultural diversity. It takes as its premise that assessment is an integral part of instruction that becomes a main component for attaining of equal opportunities. Therefore, assessment plays a key role in terms of the wider consequences at both individual and societal levels. One of the central functions of assessment is its measure of quality assurance and comparability for grading to such an extent that it is readily employed to indicate evidence of student achievement of standards and quality. This may sometimes present issues in terms of learner diversity. We focus on the challenges facing teaching in linguistically diverse learning settings in which a foreign language may be used as an alternative to instruction. Here we draw on a recent study from two separate multilingual learning contexts in Sweden. We shed light on the generic questions arising from such disjuncture in these linguistically diverse educational sites as evidence on a call for much needed scholarly attention on the quality aspect in assessment.