[...] the Messages tab's initial <StatusSelectOne> string is displayed with wrong encoding. Also the translated strings in Notices tab are displayed with wrong encoding.
I wished for UNICODE. 
That would for sure solve a lot...
BTW, I'm possibly mixing reports from two different computers

each displaying something correctly and something other not

First I've to be sure I'm not confused, then report...
For the notices, go to: C:\Program Files\eFMer\BoincTasks\html
Try changing the encoding: charset=windows-1252.
This may help. You need the restart BT for the template to be used again or to select another computer with a different notices text.
My
C:\Program Files\eFMer\BoincTasks\html\notices_template.html does already contain "charset=windows-1252".
But I've noticed that the
Notices tab contains IE context menu (IE is default on the machine) - it is set do display "West European languages (Windows) encoding". Changing it manually to "Mid European (Windows)" solves it. (Should be solved automatically by the charset preference, neeeee?? IE....)
You can use this as well: C:\Users\user_name\AppData\Roaming\eFMer\BoincTasks\html
And change the notices.html file itself, easy for checking but not permanent.
Both IE and Chrome display the encoding incorrectly: IE (like BT) as "West European languages (Windows) encoding", again a manual change to "Mid European (Windows)" solves it; Chrome as "Western (Windows-1252)", manual switch to "Mid European (Windows-1250)" solves it again.

Damn!! That's the culprit - my code page is Win-1250, not Win-1252! And Win-1252 is "Western Latin1".
I'm dumb:
Try changing the encoding: charset=windows-1252. This may help.
you wanted me actually to change the encoding from existing Win-1252, not into Win-1252 ..... Reading
and thinking simultaneously sometimes helps.
Could the
notices_template.html be somehow generated according to the selected language (using the encoding, selected in
.btlang's first line)? (This would possibly mean pushing
notices_template.html from ProgramFiles somewhere into user-writable data area?)