Each year around 10,000 people complete the Annapurna Circuit. There are plenty of lodges to stay in. The rooms are twin rooms. No, although the Annapurna Circuit, including the Annapurna Sanctuary is 22 days long, it can be done faster, but you are there to enjoy the mountain views. An allowance should be built into an itinerary for obtaining trekking permits, bad weather, illness and rest days.
Many people neglect to take rest days, and find themselves too tired to enjoy the experience.
The Langtang trek takes around 15 days. No, most Nepalese who are in regular contact with tourists speak some form of English. Nepali is similar to Hindi, rather like Norwegian is similar to Swedish and Danish. They are sister languages. Although some people do experience stomach upsets in Kathmandu, remember a Nepalese visiting the USA would have a similar problem.