I'm not certain that writing letters improves anyone's grammar. It can just as easily reinforce bad habits by repeating an error many times. Posts on the internet? You meant that as a joke - I hope. Read twitter or face book and tell me that again. The typical facebook post is questionable. Read some of the comments on this board written by Witness trained people. Writing posts has not improve their writing ability.
That your "community" publishes much literature in many languages says nothing about the grammar of your congregants, some of whom do not understand what they read. And it reinforces my comments on the narrow focus of many Witnesses. Note that I did not say the majority. Just 'many.'
And I make this additional assertion: The Watchtower fears an informed, questioning believer. They view them with suspicion.
Completing high school is not a guarantee of anything. Many U.S. colleges have sub 100 level courses to bring students up to an acceptable level. Mind you, that's not to make them grammarians, just approximately competent report and essay writers.