Decimal BASIC Windows edition deals with characters in Microsoft character encoding.
If you want to deal with Unicode characters, use Decimal BASIC Lazarus edition, of which character encoding is UTF-8.
Decimal BASIC Open Source Project Downloads
Select BASIC0651En_Win.zip.
To set the character encoding to UTF-8, the following must be written at the beginning of the program unit where Unicode characters are manipulated.
OPTION CHARACTER MultiByte
BASIC Accelerator also deals with UTF-8 characters.