> There is no way to perform case conversions and character classifications according to the locale. For (Unicode) text strings these are done according to the character value only, while for byte strings, the conversions and classifications are done according to the ASCII value of the byte, and bytes whose high bit is set (i.e., non-ASCII bytes) are never converted or considered part of a character class such as letter or whitespace.
I actually like that python doesn't do locale aware case conversions. You can use ICU* for that, though it shows more warts (like the i in republic and that you have to handle Chinese banknotes differently as well):
% env - LC_ALL=en_US.UTF-8 PATH="$PATH" python3
Python 3.8.12 (default, Nov 13 2021, 10:49:08)
[Clang 11.0.3 (clang-1103.0.32.62)] on darwin
Type "help", "copyright", "credits" or "license" for more information.
>>> from icu import UnicodeString, Locale
>>> s = b'the Republic of T\xc3\xbcrkiye'
>>> s = s.decode()
>>> s
'the Republic of Türkiye'
>>> lc = Locale("TR")
>>> s = UnicodeString(s)
>>> s
<UnicodeString: 'the Republic of Türkiye'>
>>> s = s.toUpper(lc)
>>> s
<UnicodeString: 'THE REPUBLİC OF TÜRKİYE'>
>>> s = str(s)
>>> s
'THE REPUBLİC OF TÜRKİYE'
>>> s.encode()
b'THE REPUBL\xc4\xb0C OF T\xc3\x9cRK\xc4\xb0YE'
>>> s = UnicodeString(s)
>>> lc = Locale("CN")
>>> s = s.toLower(lc)
>>> s
<UnicodeString: 'the republi̇c of türki̇ye'>
>>> s = '壹,貳,參,肆,伍,陸,柒,捌,玖,拾,佰,仟,萬'
>>> s.encode()
b'\xe5\xa3\xb9,\xe8\xb2\xb3,\xe5\x8f\x83,\xe8\x82\x86,\xe4\xbc\x8d,\xe9\x99\xb8,\xe6\x9f\x92,\xe6\x8d\x8c,\xe7\x8e\x96,\xe6\x8b\xbe,\xe4\xbd\xb0,\xe4\xbb\x9f,\xe8\x90\xac'
>>> s = UnicodeString(s)
>>> s
<UnicodeString: '壹,貳,參,肆,伍,陸,柒,捌,玖,拾,佰,仟,萬'>
>>> s = s.toLower(lc)
>>> s
<UnicodeString: '壹,貳,參,肆,伍,陸,柒,捌,玖,拾,佰,仟,萬'>
> There is no way to perform case conversions and character classifications according to the locale. For (Unicode) text strings these are done according to the character value only, while for byte strings, the conversions and classifications are done according to the ASCII value of the byte, and bytes whose high bit is set (i.e., non-ASCII bytes) are never converted or considered part of a character class such as letter or whitespace.