You can add tags by right clicking on a cel and selecting "Add Frame Tag". No tag modifying and deletion is implemented yet.
I'm using a different ScrollContainer for the tags, which gets updated everytime the main timeline ScrollContainer gets updated. I hide its sliders from its theme, wish there was a more straightforward way.
To avoid issues with the TreeItem strings being translated. Starting to get more translation-friendly again, will commit changes to Translations.pot, en.po and el.po soon.