Problem of lag while adding a language track to a video

I have recently started to add language versions to movies.
I have a movie in French and i want to add the english audio track to have two languages for the same movie.
To do that, i download the english audio of the movie which has a different length from the movie that i have on my computer.
So i changed the speed thanks to audacity, and then when the movie and the english audio had the same length, i combined them with a software.
However, when i play the movie, i notice that the lag between the english audio and the movie isn’t constant : it varies throughout time.
On the first minute, the lag would be 1 sec but on the fifteenth minutes it would increase.
Do you know why this issue is happening ?
Normally, a movie and its audio aren’t supposed to have a changing lag.
Can you help please ? How can i resolve the issue ?
Best regards

Maybe you shouldn’t change the speed of the track, maybe you should just add or eliminate silence from the beginning to make durations to match.

Do you know why this issue is happening ?

Film is 24 frames per second and PAL video is 25 frames per second so it’s often just sped-up by 4% for video. NTSC (North America) is 30 FPS so you can’t simply speed it up.

If you can confirm that (that the English soundtrack is 4% longer/slower) use the Change Speed effect and increase it by 4%.

from [u]Wikipedia[/u]

Both PAL and NTSC have a higher frame rate than film which uses 24 frames per second. PAL has a closer frame rate to that of film, so > most films are sped up 4% to play on PAL systems, shortening the runtime of the film and, without adjustment, slightly raising the pitch of the audio track. > Film conversions for NTSC instead use 3:2 pull down to spread the 24 frames of film across 60 interlaced fields. This maintains the runtime of the film and preserves the original audio, but may cause worse interlacing artefacts during fast motion.