Minor localization adjustments

[Fs00]

This PR fixes some more untranslated UI strings I've stumbled upon.
While I was at it, I took the opportunity to make some minor improvements to the GUI code:

• removed all usages of the wxT macro, which is useless since wxWidgets 3.0
• addressed a small issue in commit f6c3c96, which mistakenly added two translatable strings containing nothing or just newlines

One last thing: I added the wxString::FromUTF8 to the wxString <-> std::string conversions mentioned in the code style guidelines.
I think that it should be the preferred way to convert a std::string into a wxString. Also, wxWidgets 3.3 will add a FromUTF8 overload that takes a std::string_view, so we'll be able to use that one in place of our custom helper.