Telemark is a county in East Norway.
heavy water sabotage mission during WWII
Norwegian is the main language (but beware of difficult dialects). Almost all Norwegians speak English well enough to help tourists with anything.
By air: Telemark is served by two airports with scheduled flights, Skien Geiteryggen Airport and Notodden Airport.
Nearby Sandefjord Airport Torp  has direct flights from many European cities. From there, take a bus to Porsgrunn, Skien or Bø. You can also take the gratis shuttle bus to the nearby train stop ("Torp Sandefjord Lufthavn")  and then the train (to Porsgrunn or Skien), but this option is mostly used by people taking the train elsewhere.
By car: E18, the main road of Eastern and Southern Norway, passes through Telemark in the districts of Porsgrunn, Bamble and Kragerø. There are several tollbooth stations, but they are of course not that expensive if you can afford renting a car and paying for fuel. E18 between Gardermoen is one of the best roads by Norwegian standards. Local roads and even other E18 stretches can require both patience and experience. During the worst summer traffic, you will need 5+ hours for a 200km trip on E18.
By bus: Lots of buses from Oslo, around 270NOK/person.
By train: From Gardermoen Airport via Oslo, STADLER Flirt train sets (new 2012) go along the coast to Porsgrunn (about 2h35m) and Skien (2h45m). Tickets are 300+NOK, 199NOK ("Minipris") if ordering a few days ahead. Another, newish train set (from early '00s) goes to Nordagutu, Bø (2h7m), Lunde and Drangedal. Do not put wet luggage on the overhead, as the water will drip down on you.
By city bus Metrobuss  covers the urban parts of Skien, Porsgrunn and Bamble. A single adult ticket is 30NOK.
Ibsen's Venstøp, Skien - The place where the world-famous writer Henrik Ibsen was born.
Telemark is considered extremely safe. There is really no need to avoid any places at any time, although you should probably not provoke drunk people in taxi lines around 2AM-3AM.