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I would eliminate lines and clean up the code by relying on regular expressions.

QString s = "áche über dir Ke$ha is worth 100ドル";
// Eliminate characters you do not wish to have. Will make string smaller and improve performance.
s.remove(QRegularExpression("[" + QRegularExpression::escape("'!*,?|¡¿") + "]"));
qDebug().noquote() << "Before:\t" << s;
// Check for characters
if (s.contains(QRegularExpression("[" + QRegularExpression::escape("$/:ÀÁÄÙÛÜàáäçèéêëïñóöùûü") + "]")))
{
 // Special Characters - The Escape function is a just a safety measure incase you accidently insert "^" in the square brackets.
 s.replace(QRegularExpression("[" + QRegularExpression::escape(":/") + "]"), "-");
 s.replace(QRegularExpression("[$]"), "s");
 // Upper Case
 s.replace(QRegularExpression("[ÁÀ]"), "A");
 s.replace(QRegularExpression("[Ä]"), "Ae");
 s.replace(QRegularExpression("[ÜÛÙ]"), "U");
 // Lower Case
 s.replace(QRegularExpression("[áà]"), "a");
 s.replace(QRegularExpression("[ä]"), "ae");
 s.replace(QRegularExpression("[ç]"), "c");
 s.replace(QRegularExpression("[ëêéè]"), "e");
 s.replace(QRegularExpression("[ï]"), "i");
 s.replace(QRegularExpression("[ñ]"), "n");
 s.replace(QRegularExpression("[óö]"), "o");
 s.replace(QRegularExpression("[ûù]"), "u");
 s.replace(QRegularExpression("[ü]"), "ue");
}
qDebug().noquote() << " After:\t" << s;

Before: áche über dir Ke$ha is worth 100ドル
 After: ache ueber dir Kesha is worth s100

Oops; found an error in your code. Lets just adjust this line then:

 s.replace(QRegularExpression("[$]([^0-9])"), "s\1円");

Before: áche über dir Ke$ha is worth 100ドル
 After: ache ueber dir Kesha is worth 100ドル

I would eliminate lines and clean up the code by relying on regular expressions.

QString s = "áche über dir Ke$ha is worth 100ドル";
// Performance: Eliminate characters you do not wish to have. 
s.remove(QRegularExpression("[" + QRegularExpression::escape("'!*,?|¡¿") + "]"));
qDebug().noquote() << "Before:\t" << s;
// Performance: Check for characters
if (s.contains(QRegularExpression("[" + QRegularExpression::escape("$/:ÀÁÄÙÛÜàáäçèéêëïñóöùûü") + "]")))
{
 // Special Characters  // Escape function is a safety measure in case you accidentally insert "^" in the square brackets.
 s.replace(QRegularExpression("[" + QRegularExpression::escape(":/") + "]"), "-");
 s.replace(QRegularExpression("[$]"), "s");
 // Upper Case
 s.replace(QRegularExpression("[ÁÀ]"), "A");
 s.replace(QRegularExpression("[Ä]"), "Ae");
 s.replace(QRegularExpression("[ÜÛÙ]"), "U");
 // Lower Case
 s.replace(QRegularExpression("[áà]"), "a");
 s.replace(QRegularExpression("[ä]"), "ae");
 s.replace(QRegularExpression("[ç]"), "c");
 s.replace(QRegularExpression("[ëêéè]"), "e");
 s.replace(QRegularExpression("[ï]"), "i");
 s.replace(QRegularExpression("[ñ]"), "n");
 s.replace(QRegularExpression("[óö]"), "o");
 s.replace(QRegularExpression("[ûù]"), "u");
 s.replace(QRegularExpression("[ü]"), "ue");
}
qDebug().noquote() << " After:\t" << s;

Before: áche über dir Ke$ha is worth 100ドル
 After: ache ueber dir Kesha is worth s100

Oops; found an error in your code. Lets just adjust this line then:

 s.replace(QRegularExpression("[$]([^0-9])"), "s\1円");

Before: áche über dir Ke$ha is worth 100ドル
 After: ache ueber dir Kesha is worth 100ドル