functions.php

<?php
function tl_umlaut_repair( $content ) {
	
	/*
	 * Why? 
	 * For everyone getting this warning from W3C: "Text run is not in Unicode Normalization Form C."
	 * http://www.w3.org/International/docs/charmod-norm/#choice-of-normalization-form
	 */
	
 	/* 
 	 * See: http://www.fileformat.info/info/unicode/char/0308/index.htm
 	 */
	$content = str_replace( "a\xCC\x88", "ä", $content );
	$content = str_replace( "o\xCC\x88", "ö", $content );
	$content = str_replace( "u\xCC\x88", "ü", $content );
	$content = str_replace( "A\xCC\x88", "Ä", $content );
	$content = str_replace( "O\xCC\x88", "Ö", $content );
	$content = str_replace( "U\xCC\x88", "Ü", $content );

	/* 
	 * Requires PHP 5.3+
	 * Be sure to have the PHP-Normalizer-extension (intl and icu) installed.
	 * See: http://php.net/manual/de/normalizer.normalize.php
	 */
	// $content = normalizer_normalize($content, Normalizer::FORM_C );

  	return $content;
}

add_filter( 'content_save_pre', 'tl_umlaut_repair', 10, 1 );



function tl_umlaut_repair_title( $title ) {
    // Repair post title
    $title = normalizer_normalize( $title, Normalizer::FORM_C );
    return $data;
}

add_filter( 'title_save_pre' , 'tl_umlaut_repair_title' );