Organizational changes such as these are always driven by one or several of the following reasons:
-- Protection of organization assets
-- Protection of organizational prestige
-- Maintenance or extension of power over adherents
I don't see how closing down foreign language congregations / groups would help with the first 2 items on the list.
So it must be item 3 - maintenance of control.
That seems to fit - I recall hearing a discussion not long ago of "concern" by some elders about young JWs joining foreign language groups because it was the "in" thing to do. They were doing it because it made them (the JW kids) look good in the eyes of their peers ("yeah, look at me, I'm so spiritual, I'm pioneering in the Mandarin group") but they weren't really "putting in the work". They didn't catch onto the language quickly, or at all, and outside of 4 hours a week at the Kingdom Hall, never spoke or even tried to learn the language. Plus it was "easy field service" - go to one address where someone speaks the language, then drive half an hour to the next address, then another half-hour drive to someone's return visit....you could spend 4, 5, 6 hours a day "in service" and never have to "give a witness".
If it bugged the elders I was listening to, they certainly would have voiced their "concerns" to the CO, who would then report back to the branch. If the branch offices were getting dozens or hundreds of those reports, well, that explains it. Can't have the kids (or other publishers) getting "easy time" and mentally checking out at meetings - they should be miserable just like everyone else.