Hi Frank, I did some research and came up with following responsive solution, which fits my needs, but should be usable for all:
<video controls width=100% height=auto>
<!-- Video source -->
<source type="video/mp4" src="/videos/examplevideo.mp4">
<!-- Subtitles -->
<track src="/videos/examplevideo_DE.vtt" label="Deutsch" kind="subtitles" srclang="de" default>
<track src="/videos/examplevideo_EN.vtt" label="English" kind="subtitles" srclang="en" >
<!-- Fallback content -->
Your browser does not support HTML5 video. Here is a <a href="/videos/examplevideo.mp4">link to the video</a> instead.
</video>
Hope that also helps other users!