(covers information from several alternate timelines)
He was assigned to security and guarded the Entharan arms dealer Kovin onto the bridge where Kovin bargained with Captain Janeway. At Janeway's order, Lang then guarded Kovin on his way to engineering. (VOY: "Retrospect")
In 2375, Lang was one of two security officers that watched Tom Paris when he was demoted to the rank of ensign in the captain's ready room. He and the other security officer grabbed his arms to escort him to the brig afterwards, but Tom pulled back, saying he knew the way.
Later that month, while he was on guard in Paris' brig, the starship came under attack and the crew was called to battle station. Lang quickly ran out of the brig to his post, leaving Paris alone behind the force field. (VOY: "Thirty Days")
Later that year, he was on a security team with Tuvok sent to apprehend Seven of Nine while the crew was under the influence of a telepathic pitcher plant. She trapped Lang, and the team, behind a force field in engineering. (VOY: "Bliss")
Later that year, Lang went on an away mission with Harry Kim and The Doctor to rescue what was later found to be a Druoda missile. When Kim and The Doctor needed to decide whether to beam it back to the ship, Lang was told to keep the warhead "company" and watch it. (VOY: "Warhead")
In early 2376, just after the USS Equinox left Voyager to fend off the nucleogenic lifeforms without a shield generator, he was on guard with a compression phaser rifle in sickbay when Captain Janeway came in with The Doctor's mobile emitter. (VOY: "Equinox, Part II")
A hologram of Lang appeared in the holoprogram The Voyager Encounter, where he was present at the meeting with Ambassador Vaskans and the Captain Janeway after the capture and killing of the Kyrian Tedran. When Tedran refused to surrender, Janeway took Lang's compression phaser rifle and used it to kill both Tedran's accomplice and Tedran, before tossing the rifle back to Lang. (VOY: "Living Witness")
Lang was portrayed by regular background actor Sylvester Foster who received no credit for his appearances.
He was identified by name as "Crewman Lang" in the episode "Warhead". This character's first name and enlisted rank of "technician" were on the "Imperfection" graphic casualty list, not in any of his actual appearances.