I've realized that the App Store interface doesn't let me uploading app preview videos in a language other than the main language of the app. Really? Am I missing something?
If it's like that, it doesn't make sense. Every person needs to see the video in his/her own language! Why limit the languages in the app preview video to just one?
Is there any workaround for this? We already filmed 30 second videos with an actress talking in different languages. We never imagined Apple would impose such a dumb limitation.
At the current time it doesn't seem possible to specify a localized App Preview.
From Apple's iTunes Connect Developer Guide documentation:
The app preview you upload will be visible for all localizations.
and this from Apple's App Preview developer overview page...
Consider that your app preview is available in one language worldwide. Narration may not be appropriate in apps marketed globally. If you decide to use narration in your app preview, we recommend working with a professional voiceover actor. This is the voice of your app—make sure it resonates.
You can however specify localized "metadata, keywords and screenshots".
From the documentation:
Displaying on the Store in More Than One Language (Optional)
After you create an iTunes Connect record for your app, you can add the information for your primary localization and add additional localizations.
For example, if you created the iTunes Connect record for your app with the primary language English, stores in all territories display information for the app in English. If you now upload metadata, keywords, and screenshots in French, users whose language is set to French or who are located in a territory that includes French (and not English) as a supported language see the French metadata instead of the English metadata. Your app is also searchable in all French-language stores by the localized keywords. In other store territories, the primary language, in this case English, appears.
Without seeing the content of your App Preview (or knowing more about your app), it is hard to make further suggestions. That said, the one's I can think of are really tradeoffs and would not utilize the multiple videos that you have already made.
Possible Tradeoffs:
Choose the video most representative of your customer base. For example, if you primary customers are in Germany, choose the German video.
Replace the voice narration with text annotations (possibly displaying the text in different languages in the same scene)
Use localized screenshots
Choose one video as your App Preview (for all localizations) and host all of the videos on a website and provide the link under "Developer's Website"