Tuscany using public transportations

You need to research bus schedules carefully. Frequency is greatly reduced or non-existent on Sundays, and the times can be erratic (i.e. if there are "four buses a day," they can be a 7:30 AM, 8 AM, 10 AM and 3 PM, with nothing later, leaving you potentially stranded). As long as you're prepared for these issues, your plan is fine.

Will everyone you meet speak English? No. But in the more touristed towns, there will be enough English for your needs. The further you get off the beaten path, the less English you will encounter. You should not expect to find English menus in restaurants, for instance.

You can do it, but it takes time and good planning. One town per day would be possible, two if they are on the same train line and you start early. Don't try multiple towns on a Sunday, a roundtrip may not be possible.

Not speaking Italian shouldn't be an issue, but learn some basic travel words, like fermata, biglietto, partenza, arrivo, and the names of the cities in Italian. These words will appear on posted schedules. Also important: feriale (weekday) and festivi (Sundays and holidays).

Thank you both for your encouragement. If I'm not mistaken, the bus schedules are not available in English, so I'll work on learning some basic Italian. The one two-stop outing I hope to do is Pienza and Montepulziano.

Yep! Planning is key. We have done exactly what you're doing - staying in Siena and taking buses to neighboring towns - it worked out fine for us! The most important thing is to know when the last bus of the day is that goes from your destination back to Siena...you don't want to get stuck somewhere overnight! Schedules change dramatically during the weekend so make sure to check carefully if you're traveling on a Saturday or, especially, Sunday. Public buses there are set up mainly for commuters and locals so don't expect them to accommodate a full-day visit to an outlying town, returning to Siena late at night.
You really only need to know a couple of words in Italian to interpret the bus schedules and they are written out in pretty much the same fashion as bus/train schedules anywhere. We found them to be very intuitive. As with most rural areas, some places are easier to get to than others on public buses. If you are patient and plan ahead though you'll be fine.

It can be done but you have to do some research and planning. Some towns you can get a direct bus like San Gimignano. Others you have to change buses someplace. Based in Siena you would be using SITA to get from town to town in that part of Tuscany. This is their website http://www.sienamobilita.it/EN/index.html. Now the schedules can be found at this link. You can see all the bus number routes and where they go to. http://www.trainspa.it/train04/extraurbano.pdf

You've had some good advice. The end of Rick's Siena chapter has a quick summary of bus frequency to the closest tourist destinations. For instance, Volterra is one of those just served 4x/day, Mon-Sat. Buses may depart Siena from Piazza Gramsci or from the train station.