Irish and Breton are both Celtic languages but unity has vanished resulting in deep linguistic differences. But the common heritage is still at hand when one considers the lexicon, grammatical peculiarities and number. The concept of the structure of a language impacting on thought processes is referred to as the linguistic-relativity hypothesis, which proposes that the vocabulary and phraseology of a particular language influences the thinking and perception of speakers of this language, and that each language will have a different cognitive system. This paper examines the Irish and Breton languages in their bilingual context, their linguistic characteristics and impact on mathematics learning in comparison to English and French, while identifying future research requirements.