In practice, the foreign subtitles do not line up with the foreign words as well as the native subtitles line up with the native words. It took me years to figure out why this was.
In a conversation I had with someone who knew the business, the new language subtitles don't line up with the new language audio because the new language subtitles are translated from the original language subtitles, not the new language audio.
Why? This saves a step from someone having to rewatch the final new language dialogue and transcribe the new language as it is exactly spoken. That step can be skipped and the new language subtitles can still be translated from the original language subtitles, although they will be quite off much of the time.
In a conversation I had with someone who knew the business, the new language subtitles don't line up with the new language audio because the new language subtitles are translated from the original language subtitles, not the new language audio.
Why? This saves a step from someone having to rewatch the final new language dialogue and transcribe the new language as it is exactly spoken. That step can be skipped and the new language subtitles can still be translated from the original language subtitles, although they will be quite off much of the time.