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 either the gratis shuttle bus to the nearby train stop ("Torp Sandefjord Lufthavn")  and the train to Porsgrunn or Skien.
By bus: Lots of buses from Oslo, around 270NOK/person.
By train: From Oslo, one train (usually 20+ years old set with gratis wi-fi (but you need to register account online), but sometimes a very old set) goes along the coast to Porsgrunn (3h30m) and Skien (3h45m). Another, newer train set goes to Nordagutu, Bø (2h7m), Lunde and Drangedal. Do not put wet luggage on the overhead, as it 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.