Localisation German: jquerys’ DatePicker “ä” is not recognized

I have an odd behavior:
I use datepicker. It works fine. As long it is in English. I want it in German. Ok, reading reading reading, I found out, that I had to include a localisation file.

Januar, Februar works fine. But März don’t work. I got a “?” inside a Rhombus. More reading. Checked the files to utf8, ok.

Inserted ` No change.

Just for fun I added the french localisation. Eh voila. März looks like it should be. But the file blocks previous month.
I changed min-month from 0 to -2 and -2 M and -60 d.
Now I could go in the past, but the actual month is displayed correctly only once, and when going back and forward, it fall back to english. Neither French nor German

Then I tried another localisation file. Same problem.
Here some code:

<script src="./js/jquerymini.js" type="text/javascript" charset="UTF-8"></script>
<script src="./js/jquerymini2.js" type="text/javascript" charset="UTF-8"></script>
<link rel="stylesheet" href="../maincss/jquerycss.css"  type="text/css" media="all" />
<script src="./js/jquery-datepicker-de.js" type="text/javascript" charset="UTF-8"></script>
<link rel="stylesheet" type="text/css" href="../maincss/datepicker.css">

and here the form:

include_once('includes/header.php');
<table><td>...
<script type="text/javascript">
$(function() {$("#datepicker<?php echo $anzahlteileinsaetze;?>").datepicker();});
</script>
</td>
</tr>
<tr>
<td class="datenzelle2">Datum des Teil-Einsatzes <?php echo $anzahlteileinsaetze;?></td>
<td class="datenzelle2">
<?php echo '<input type="text" name="datum[]" size="75" value="'.$value;?>" id="datepicker<?php     
echo $anzahlteileinsaetze;?>"  class="textfeld" readonly></td>
</tr> 

There are more than 1 datepicker. It is like: basket1 on 2024/03/01, basket2 on 2024/03/03, basket on 2024/03/08

So the var is date1,date2 date3, …
The whole file is php and utf-8.
Any idea
regards
Oliver