and there are is no language barrier for prisoners and staffIn the case of Belgium and The Netherlands, there probably is. A sizable minority of Belgians speak French, not Dutch, and due to poorer economic prospects they might also make up more than half the prison population.
But I assume that for people arrested for crimes in the Dutch speaking part, being jailed by someone you can't understand is Your Own Damn Fault. I mean, if they arrest Eastern Europeans then what are they going to do? Get prison wards to learn a bunch of Slav/Russian dialects?
Edit: Checking the facts, over 60% of the prison population is not actually Belgian, and this EC report: http://www.cpt.coe.int/documents/bel/2012-19-inf-eng.htm
"34. At the time of the 2009 periodic visit, the Belgian authorities had stated that, in principle, French-speaking prisoners were not considered for transfer to Tilburg Prison. Two years later, it has to be said that almost 20% of the prisoners - Belgian or foreign - in Tilburg are French speakers. Consequently, a not inconsiderable proportion of the prison's inmates find it difficult to communicate on a day-to-day basis with the Dutch staff, most of whom speak little or no French. The delegation also noted that the instructions on what to do in the event of fire breaking out, displayed in the cells, appear in Dutch, Arabic, Turkish and English, but not in French. Such communication difficulties are not without significant consequences during official administrative procedures, particularly of a disciplinary (see paragraph 36) or medical nature. The delegation took note of the efforts made by the Dutch prison staff to improve their knowledge of French. However, the CPT invites the authorities to limit as far as possible the transfer of French-speaking prisoners to Tilburg Prison. "
highlights that the French speakers are out of luck although reasonable efforts are done for them.