Invisible column header after inserting {mlang_message tag}

In the Visual Editor I have begun replacing the German column headers with multi-language messages (like {mlang_message tag="Referenzzeit"} ) and adding the corresponding entries in "languages.php" (like $mlang_messages["English"]["Referenzzeit"]="Reference time"; and

$mlang_messages["German"]["Referenzzeit"]="Referenzzeit"<img src='https://asprunner.com/forums/file.php?topicimage=1&fieldname=question&id=5794&image=1&table=forumtopics' class='bbc_emoticon' alt=';)' />.
The problem:

Before my changes the Design Mode of the Visual Editor looks perfect, like this

Then I switch to HTML Mode, delete the column header "Referenzzeit" and add the mlang_message tag instead:
[codebox] <TD>

<TABLE>

<TBODY>

<TR>

<TD class=blackshade><A class=blackshade

href="dungturnier_list.php?orderby={$order_dir_Teamname}Teamname">Teamname</A>

</TD>

<TD>{$order_image_Teamname}</TD></TR></TBODY></TABLE></TD>

<TD>

<TABLE>

<TBODY>

<TR>

<TD class=blackshade><A class=blackshade

href="dungturnier_list.php?orderby={$order_dir_Referenzzeit}Referenzzeit">{mlang_message tag="Referenzzeit"}</A></TD>

<TD>{$order_image_Referenzzeit}</TD></TR></TBODY></TABLE></TD>

<TD>[/codebox]
after that change is applied the Design Mode does not display the column header any more:

but the output works fine on the actual web site. I fear that by replacing ALL column headers it will render the Design Mode useless...
Any suggestions as to what is wrong with my code? Or is it a bug?
Greetings,

Wilfried