What's deal here? Assimilation or how good a job they're doing? If it some menial, repetitive job, like bring Box A to the place B, their command of the language needn't be anything special.
If it's some job you need qualifications for, like some managerial job - well, in that case those who don't speak the language (or are unwilling to learn it) generally won't be interested in the job, because in order to do that job well, quality of communication needs to be high.
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