I have not seen this one before. It could be a duplicate.

When entering a separate language using the (i.e., <span lang=ja>), the language filter works OK until I use the HTML editor to mark up text (make bold, h1 etc.)

It seems that the closing </span> tag from the regular text markup nullifies the language filter tag.

Changing the HTML code to regular tags (<b>text </b>) seems to work.