For the geographically challenged, I should perhaps mention that Slovenia is a first-world country. It is, however, less expensive than the States. A bit of German or Italian may be useful but isn't essential. Bus conductors seem to tend more to German than to English, but in any case you can get by with nothing more than place-names and gestures. In hotels, restaurants, and other businesses, you can almost always use English.
Temperatures are usually pleasant during the day but can be nippy in the morning and evening.
I advise bringing a sweater or light jacket. If you plan to hike in the nearby mountains, bring warmer clothing. Like all weather forecasts, this one should be taken with a heap of salt. Beyond this, you're on your own. Bled and its surroundings are beautiful.