[ymca]

MKV with build-in subtitle.

Likely NOT. If the subtitles are encapsulated into the video, they become part of the visual image, no further easy manipulation can be done. If alternate languages are offered, then the subtitles are part of the MKV container and could be extracted MKVToolNix (wiki) for use as external subtitles and at that point, font, colour, and position would be accessible. 

I can enable or disable the subtitle.
But I didn't find a way to move it to the bottom.
Only when I download a subtitle from the internet I can get it to show at the bottom of the video.

[jjd-uk]

Then it's likely the mkv contains image subtitles, so in the case of a Blu-rip these would likely be PGS subtitles which can't be manipulated, only text subtitles such as SRT can be manipulated.

[sergejskr]

Its obviously a common bug, also it would be much easier to alter those settings while playing the movie and not going into the system menu and altering settings. The great example how it should be implemented can be found on Zidoo native players, they've just nailed it and it works as supposed to.