I am still not sure whether this is a bug, or was intentional. Nothing can be seen in weekend meetings when the info language does not match the UI language.
So for talk themes, this means they need to be defined as being in the same language as your user interface.
So if you use English as your UI language, you will see the themes only when TB thinks they are English. To fix this there are essentially two options:
Import the talks again and define the talks as in a new language.
Probably better: the database outside of the programme itself. Identify the language ID of your user face language in the 'languages' table. Then in the 'publictalks' table replace each occurrence of lang_id with that ID. If you know a bit of sql, you could do it through a formula, but that is beyond me. If you do it manyually it is quite boring, but it will take 5 minutes or so, which may even be less than a reimport.
As for the Wachtower study, you can enter the info manually. See this post in an otherwise unrelated issue: https://www.theocbase.net/support-forum/post/8083-bug-import-w_i_202106.html#reply-8086