In the final verse of "Old MacDonald" the banners for each scout group switch from being the names of nations to being nationalities (e.g. the first two verses show signs like "France" and "Japan" and the last verse the signs become "Mexican" and "Dutch").
There are no Scoutmasters from those other Countries to lead the other troops. There are clearly taller scout troops sitting with their respective countries during the singalong, which would clearly be the Scoutmasters.
When their getting to Shangri La-De-Da Valley they hike the rest of the way on foot but when the tents are up you see both of the families cars.
When the scouts cheer their troop leader, they call him Mr. Sandstone. When Fred talks to him, he calls him Mr. Jones.
Fred technically shouldn't have pointed to England during "Old MacDonald Had a Farm" if he wanted scouts in other non-English speaking countries to sing the song in their language. All the scouts from England did was sing in English, but with a British accent.