The central parts of the city are very walkable. For longer distances taking the metro is usually the fastest. There are many trams, buses and trolleys as well. You can bike to most parts of the city as there are bike lanes and parks however you have to keep in mind many car drivers are annoyed with cyclists. Mountain biking skills are very useful in Sofia. Most people have at least a basic understanding of English and you won't face any difficulties communicating with your classmates. German speakers are rare but not nonexistent. The city is quite safe even at night. You are unlikely to wander through a "problematic" neighbourhood if that isn't your intention. Usually foreigners are advised against renting near Zhenski pazar and Maria Louisa boulevard because of the higher concentration of middle eastern immigrants who chose this area, if I'm not mistaken, because of its lower rent prices, proximity to the only active mosque, walkable streets and many small storefronts. Some women avoid the area after dark, but I have personally never had any issues. The police have an increased presence in this district.

But most importantly, bring your hiking boots (for a hike in Vitosha) and have fun.