Throughout our code the callers of QLocale APIs that I upgraded from returning QChar to returning QString at 6.0 may still be assuming single-character values.
Examples:

• qtbase/src/widgets/widgets/qspinbox.cpp comparing copy.at(0) == locale.decimalPoint() and similar. (QTBUG-138521)
  • Same file, later using dec = copy.indexOf(locale.decimalPoint()); but later assuming dec + 1 is where to look for what follows it.
• Qt-dev/qtbase/src/corelib/serialization/qtextstream.cpp comparing data[0] to negativeSign() and positiveSign() returns. mmutz brought this to my attention, sparking this ticket: he is fixing it. (QTBUG-138484)
• Same file, QTextStreamPrivate::getReal() looping while (getChar(&c) and assuming it can compare c.toLower() – itself potentially problematic – with the toLower() of various QLocale no-longer-single-char values. It needs to use case-insensitive starts-with tests on the stream of characters and consume them chunkwise.
• qtdeclarative/tests/auto/qml/qqmlvaluetypes/tst_qqmlvaluetypes.cpp testing QCOMPARE(object->property("decimalPoint").toString().at(0), locale.decimalPoint()); and similar for the other functions, rather than doing a startsWith() test.
• qtcharts/src/charts/axis/valueaxis/valueaxislabel.cpp testing event->text().at(0) != locale.decimalPoint() and similar for negative sign (neglecting positive) and exponential; also ignoring group separator; and testing event->text().at(0).isDigit() assumes BMP digits.

Scan code for calls to these functions

    QString decimalPoint() const;
    QString groupSeparator() const;
    QString percent() const;
    QString zeroDigit() const;
    QString negativeSign() const;
    QString positiveSign() const;
    QString exponential() const;

and check for any callers assuming single-character returns (like they used to get in Qt 5). For now (with CLDR v47's data) every locale has single-UTF-16 fractional-part (misnamed "decimal point") and grouping separators, so fixes for the first two are relatively low priority; but all the others are multi-UTF-16 in at least some locales.