in includes/search_utils.php find:function convert_special($text) {
Insert below: return $text;
It wont fix the problem right the way, because the latin letter in your keywords are already saved into the database, so you'll need manually edit database and fix these keywords (if you manually add keywords to each image, you can edit image and fix keywords there).
P.S.
I can't guaranty that it will work properly, but you also can try this before you manually edit database:
in includes/functions.php find:function get_tag_clouds() {
Insert above:function _convert_special($text) {
return strtr(
$text,
array(
"AE" => "Ä",
"OE" => "Ö",
"UE" => "Ü",
"ae" => "ä",
"oe" => "ö",
"ue" => "ü",
"ss" => "ß"
)
);
}
find: $tags[$row['word_text']] = $row['quantity'];
Insert above: $row['word_text'] = _convert_special($row['word_text']);