How to recognize speech bubbles in comic strips (Ideally OpenCV)
As said in the comments an efficient way is to first detect letters, words and text with OCR. Then try to expand each text zone to its corresponding text bubble.

Depending on the text bubble design there are different approaches. However, a solution that could work well and be robust would be to perform edge detection on the near surrounding of the previously detected text zones and then keeping only the (radially) closest edges from the text zones(that should be the bubble borders).

This is just a basic idea, you may have to build a more sophisticated algorithm obviously!