Perhaps there's a U-curve. When I worked in a hotel in a metropolitan city, almost all workers there were multilingual. People from lower-income countries tended to do the unskilled work, whereas people with higher education tended to work at the front desk and in office roles, both having a mix of temporary and permanent residents.

It makes sense to me that the great in-between has less incentive to learn another language, if they're in a monolingual milieu. They don't have the economic push to find better opportunities elsewhere nor the means (or at least the idea) to travel and explore.