Hover Translate/Hover Text
It's still possible to do this, though it's a little more tricky/confusing since the layout changed (yet again).
Firstly, using whatever means you prefer to translate - for me it's usually google translate - paste/type your text:
Next of all, highlight the text you want to be translated:
Click on the 'insert link' button and mark where you want the translation to go. Past or type in the translation or the text you want to appear when you hover over the words:
Go to the gear icon/settings in the upper right of the text box, click on it. Move down to the 'text editor' option and click on it. Select 'HTML'.
* It should be noted now that if you wish to include icons/gifs/pictures in your text post, do so before changing things to HTML.
The text post is now in HTML. This is where tumblr changed things a little. All along, there was a 'title' part of the code along with the link part of the code - 'href'. There is no longer a title part, which was the part we needed to create the hover translation/text.
Fret not, that can be fixed.
Go to where you typed in your translation/text you want to make hover:
Highlight the 'href' part and replace it with 'title' instead.
You can go ahead and press the post button now :)