Thanks for the reply. Here is my research on the matter:
Since RDS2 from 2015, text can be encoded in UTF-8: Radio Data System - Wikipedia
Even before that, the encoding wasn't limited to ASCII as the EBU Latin (see below) code table was similar to ISO-8859-1 and did contain accented and other characters used in Western European languages.
References: ts_101756v010601p.pdf
Page 9, section 5.2, Charsets. Old EBU Latin, UCS-2 and UTF-8 can be used.
Page 42, Annex C, see the EBU Latin table.
So the problem is this: The Samsung Radio app almost certainly expects UTF-8. The broadcaster may correctly send EBU Latin, but without the indicator as per section 5.2. Note that any highbit characters (>0x80) are invalid code points in UTF-8 and replaced with the UFT-8 replacement character � (0xEF 0xBF 0xBD).
I'm actually a software engineer with a good understanding of charset encodings. So the only way to solve this would be to get into touch with the app developers at Samsung.
I was in Germany recently where they also use RDS. Foolishly I didn't turn the radio on there ☹️ - Anyway, perhaps someone from a "non-ASCII" country could tell us whether the Samsung Radio app works in their country and displays non-ASCII text there. Someone from Germany, France, some Eastern-European country or Latin America. Surely the Europe-centric EBU charset doesn't allow to encode Korean, Japanese or Chinese text, so maybe someone in those countries can add their experiences here.
I'm also trying to get in contact with the Spanish state broadcaster, but so far after two months of trying, they've just ignored the issue.
EDIT: BTW, my old Nokia 1 leaves out the non-ASCII characters instead of displaying a � - So unless the Nokia radio app has a similar bug, that hints at the broadcaster not transmitting correct data.
