The thesis defended in this paper can be summed up as follows: it seems to us that the decline of the Breton language can be explained by economic reasons from the inter-war period onwards. It was at this time that the Breton economy moved from a self-sufficient economy to a monetary economy. One of the aspects of this transformation (which is undoubtedly not yet complete) was the confrontation of two different systems of evaluation: each economic act, such as production, consumption and work, had its own mode of evaluation in each system. The study of the Breton language should provide evidence of this evolution. Based on the cases of 1) the transition to monetary exchange, and 2) the transition to salaried work, we will attempt to set out a possible problem.