Maybe you can get the Bengali leaders of the AIML like Suhrawardy and A. K. Fazlul Haq to threaten Jinnah and other pro-Urdu politicians that Bengalis would make up the majority in Pakistan and that they would not be happy with the proclamation of Urdu, and only Urdu, as the official language and as such they would split from the Muslim League and keep winning elections on the backs of Bengalis? That could stop the language movement. Language was not the only reason East Pakistan left, however, the main issue was the economic disparity between the two wings. Maybe getting more Bengalis as PMs of Pakistan for long periods of time would help.