The episodes "Double Geopardy" and "One Minute to Midnight" were produced in the context of the 4th season but were only broadcasted in the United States (origin country of this series) during the 5th season. For this reason, these 2 episodes are listed in the 5th season.

This is not accurate. The Highlander TV series was made for broadcast in the UK and was broadcast in the UK before being broadcast in the US. It released in both countries on the same day(3-Oct.-92), but technically aired in the UK first because of the 5 hour time difference. The episode order used on IMDB is the episode order from the VHS release of the show, which is the original UK broadcast order. A few episodes were broadcast out of order in the US due to shipping issues with the broadcast tapes. The UK order should take priority as it is 5 hours ahead of the US. In the UK the 2 episodes in question aired during series 4 as the 19th and 22nd episodes respectively. Of course this also means the entirety of Season 5 is incorrectly numbered.

FTR, I have been trying to get IMDB to correct the air dates for those episodes, as they are using the UK broadcast order but the US air dates.

As explained here, the series was created to be broadcasted in the United States :

• The French leading production company Gaumont Television bought the rights to the series to have it produced in syndication in America with a local crew, a ground-breaking move at the time.
• Highlander marked the first time a French production company was creatively involved in a show intended for the American market.

Furthermore, there are no United Kingdom production companies credited.

Consequently, the origin country of this series should be the United States.

By the way, the United Kingdom broadcast has only started in April 1994 (more that one year after the United States broadcast) as indicated here.