You need a good quality 1080 HD copy of the original master video, with the English voiceover on one audio channel and the music and background sounds on the other.
This is described as having a separate “M & E” (music and effects) track.
What happens next is that the English voiceover will be used as a guide for the new language, which will then be laid down in its place.
The original music and effects will be kept.
If the M & E has been mixed in with the English voiceover, then you will need to create a new M & E track.
Unless the background sounds are very distinctive, this can usually be done in the studio after recording the new voice, using stock sound effects.
Depending on how any background music has been licensed, you may also need to pay additional fees to use the same music on your foreign versions.
There are situations where the English is spoken quickly, while the required foreignspeak is necessarily slow (eg Finnish).
When this happens there simply isn’t the time to lay the foreignspeak into the available space.
The solution is to re-edit parts of the original video to make it fit. Consult with an experienced editor on this.