yes. Usually, Regine (the girl singing) is the one singing in French. Parts of the songs Empty Room- The Suburbs, Haiti- Funeral, Un Annee Sans Lumiere(a year without lights)- Funeral, are songs with French Lyrics.
Easy songs to sing are songs by country pop singer Taylor Swift. Her range is small in most of her songs and they sound super nice when you sing them. Others are songs like "Only Exception" by Paramore. I find that song simple to sing. Selena Gomez's songs are super easy to sing, too.
The easiest songs for children to learn and sing are happy and catchy songs. Songs dealing with Christmas are ones that children learn fast and sing. 'Rudolph the Red Nosed Reindeer' and 'Jingle Bells' was popular Christmas songs.
In musicals, yes, the characters do sing the songs. In stage productions, it is possible that the characters do sing the songs. In movies that are not musicals, then it is likely that any song heard is background music, and is not sung by the characters.