Who is needed to support this strategy (e.g., Wikimedia Foundation, chapters, individual volunteers, external partners), and what do they need to do?

Wikimedia Foundation

I think it is the foundation's task to encourage the chapters to formulate SMART goals for leveraging the local languages within their geographic area, as well as setting those goals itself - including fund allocation - for areas that are not covered by chapters. The foundation is IMO also responsible for aggregating the results that are reported from the chapters, so that a global overview of achievements and progress will be available, and best practices can be drawn up based on the successes and failures in the past.

chapters

The chapters are closest to the individual volunteers and local external partners. This process will only scale if the chapters take the responsibility of communication, coordination and execution of set goals together with the individual volunteers, local external partners, and international external partners where applicable. Monitoring, tracking, and chasing should be done on this level, because doing this on a global level will mean that a lot of language barriers have to be overcome, making the process cumbersome.

individual volunteers

Active volunteers are the back bone of everything that gets done. They are the people that care, for whatever reason, and put their time into "making things better". They should know who to approach - requires at least documentation of project goals if the volunteer wants to work within a project's scope - and they should be able to make a career in the projects; delegate responsibilities as soon as possible as low as possible to motivate the volunteers and to keep things scalable.

external partners

<no time to go into this yet>