> For me, it prints this: (includes “te” with an umlaut)
I got this same result when testing on Android with the interactive interpreter.

See the content of the 'completer' list below, when run with tracing_the_completer.patch:
On linux:
test_nonascii (test.test_readline.TestReadline) ... bytearray(b"^A^B^B^B^B^B^B^B\t\tx\t\r\n[\xc3\xafnserted]|t\xc3\xab[after]\x08\x08\x08\x08\x08\x08\x08text \'t\\xeb\'\r\nline \'[\\xefnserted]|t\\xeb[after]\'\r\nindexes 11 13\r\ntext \'t\\xeb\'\r\nline \'[\\xefnserted]|t\\xeb[after]\'\r\nindexes 1113\r\nsubstitution \'t\\xeb\'\r\nmatches [\'t\\xebnt\', \'t\\xebxt\']\r\nx[after]\x08\x08\x08\x08\x08\x08\x08t[after]\x08\x08\x08\x08\x08\x08\x08\r\nresult \'[\\xefnserted]|t\\xebxt[after]\'\r\nhistory \'[\\xefnserted]|t\\xebxt[after]\'\r\ncompleter [\'t\xc3\xab-0\', \'GOT 1\', \'t\xc3\xab-1\', \'GOT 2\', \'t\xc3\xab-2\', \'t\xc3\xab-0\', \'GOT 1\', \'t\xc3\xab-1\', \'GOT 2\', \'t\xc3\xab-2\', \'t\xc3\xabx-0\', \'GOT 3\', \'t\xc3\xabx-1\']\r\n")

On Android:
AssertionError: b"text 't\\xeb'\r\n" not found in bytearray(b"^A^B^B^B^B^B^B^B\t\tx\t\r\n[\\303\\257nserted]|t\x07\x08\x08\x08\x08\x08\x08\x08\x07\x07xrted]|t\x08\x08\x08\x08\x08\x08\x08\x07\r\nresult \'[\\xefnsexrted]|t\'\r\nhistory \'[\\xefnsexrted]|t\'\r\ncompleter [\'\xc3\xafnse-0\', \'\xc3\xafnse-0\', \'\xc3\xafnsex-0\']\r\n")

b'\xc3\xab' is the UTF-8 code for '\xEB'
b'\xc3\xaf' is the UTF-8 code for '\xEF'
It seems that the completer is triggered by the wrong key.