Thanks for the fast reply! I figured it out though..
My bad.. as I was expecting to see the “Globe” icon next to my TEXTAREA and ignored the fact that I can write translation tags manualy!
I simply added the translation tags inside the TEXTAREA like so:
{:en}English Title{:}{:de}German Title{:}
…and when displaying it on screen:
<?php echo apply_filters('the_title', get_metadata('post', get_the_ID(), 'my_custom_textarea', true) ); ?>
Cheers!