For you Apple guys, maybe you could improve a little bit the media manager tool to allow reuse of any media for any language.
"I agree it's really painful - I thought that deliver module from fastlane would automate this but it is broken with the update of itunes connect. I checked transporter (http://help.apple.com/itc/transporteruserguide/) but I don't see anything about uploading previews...
"App previews can be delivered only in iTunes Connect" - hope it has changed (since fastlane was able to do it before when the preview was not localized)
Thanks for your answer. On my side I could get things better:
1) I did a test to see if I had to upload a video for each language. The answer is NO. These days, the default language app preview is taken if you don't upload an app preview for a given localized language.
2) I realized that the "ProRes 422 HQ only" Codec generates files about 400-600 MB ... So, within Final Cut Pro X, I went through the Destination entry menu, to change the "Master Files" Codec to H.264. Then the video weight 20-40MB and it takes about 3 minutes to upload on iTunes Connect.
Hope this will help someone. Tell it if so.