If you are updating languages in addition to the Stock English language (ie; multiple languages), and everything checks out successfully, you'll get stuck at a page which says, "[LANGUAGE] language files have been updated." The bug in this situation is that there should be a button at the bottom which says, "Continue The The Next Step" or at least a button that takes you to the "Test Again" page. But there is none.
To get around that, use your browser's BACK button, and then click on the "test again" link at the top.
Now the upgrader will check your language files once again, and confirm that everything is good. There is a "Continue The The Next Step" button at the bottom of this page. You can now continue with the upgrade process as normal.
This is a display bug that I just came across after reading your initial post above -- thank you for pointing this out
The upgrader is designed to not let you continue if all your language files have been upgraded. WHY? Because if you continue with broken languages and missing strings, you'll probably also have a broken forum.
The red highlighted language strings on that initial page are lines which should be copied directly from the stock language files, and placed in to your custom language files. They should not be copy/pasted from the report screen. Especially since the report screen transposes "HTML code" in to "Executed HTML."
Therefore, if you have custom languages, its best to;
1. Make note of the strings and which files they came from.
2. Let the upgrader make the changes to your custom language files for you.
3. Finish your upgrade.
4. Go back, at your leisure, and manually update your custom languages using the reported items from that window. Not copy/paste from the report. But use the line items as a guide/reference to whats needed. OR go in to each language file and update them as you see fit. You can edit the languages either by opening the files directly, or by using the built-in Languages Editor from within the UBBT Control Panel.