What is the difference between assimilationist and monolingual policies?

The difference is in degree and in the open disregard for international instruments regarding minority rights. Most of the governments in the world adopts a monolingual policy which is mildly assimilationist (in the sense that there is that implicit coercion that the rest of the population use the language of the dominant group). However, they are careful not to blatantly violate existing international instruments on the rights of minority groups.