First of all, a disclaimer: while you're probably right that flying to Hahn is the cheapest option, it almost certainly isn't the easiest option. If getting to and from Hahn airport sounds too awful to you, consider paying a bit more to fly to Paris (from where you can take a bullet train to Saarbrücken in under two hours) or to Frankfurt am Main (there are trains that go from Saarbrücken direct to Frankfurt airport in two and a half hours; you'll pay around €30 one way).
I lived in Saarbrücken as a student for four years, and often traveled to Hahn for RyanAir flights. Unfortunately, Hahn is in the middle of nowhere (and "Frankfurt" is a total misnomer), and the airport is quite hard to get to. For a while there was a fairly convenient direct bus connection from the train station in Saarbrücken to Hahn airport, but this stopped running about a year ago. Now you're left with a set of not-so-appealing options for travel between Hahn and good ol' SB. You will have to take a bus for part of your journey.
The go-to webpage is Hahn airport's list of bus connections. The map there is clickable, and will show you current bus schedules and prices from the various operators that serve the airport. The other page to look at is Deutsche Bahn, as mentioned by another poster here already.
Basically, there are four options that make any kind of sense. The travel times and prices I'm giving here are from the last trip to Saarbrücken I did (in early October), so your mileage may vary.
- Travel via Idar-Oberstein. The cheapest option, fairly direct, takes about four and a half hours in all. There's only four or five buses a day, so you spend time waiting at the airport for the bus (€7.70 one way) and in Idar-Oberstein waiting for the train (€13.90).
- Travel via Trier. Slightly more expensive, and slightly faster, takes about four hours. There's seven buses a day on this connection (€12 one way), and more trains running through Trier (€15.80). Maddeningly, the airport bus drops you next to the Autobahn (the stop is called "Trier-Nord, Nells Park"); to get to the train station, you can choose between a 2.2 km walk (about 30 minutes), or a 15 minute city bus ride (seems to be lines 30 or 86, but you can check the website yourself).
- Travel via Bullay (Cochem). Never tried this one myself, but it's attractive because there's so many buses running that you might be able to do the trip in three and a half hours. The bus to Bullay is €6.80 one way, and the train from Bullay to Saarbrücken is about €25.
- Travel via Mainz. This makes almost no sense at all, except if you're arriving or leaving really early in the morning or really late at night (quite conceivable if you're using cheap flights). The trip will take you at least five hours, I think, and you'll pay €12 for the bus and €28.50 for the train. Personally, I'd rather sleep in the airport, but to each his own.
As for your other questions, I wouldn't worry at all. While people in the Pfalz and the Saarland might speak less English than in other parts of Germany, you'll often find somebody who can. If you have a few words of German, that'll help a whole lot (for instance, if you're trying to find the "Bahnhof"). Trains in Germany are brilliant: they're usually on time, and I think they are a bit cheaper than trains in the UK. The touchscreen ticket machines in train stations can print a travel itinerary for you with times and platform numbers. Of course, don't sit in first class if you've bought a second class ticket.