I'm assuming you mean it needs to rewritten since Member Groups isn't a maintenance item?

In %conv2x_txt:

'finish' => q~<br />Furthermore, we strongly recommend to run the following "Maintenance Controls" in the "Admin Center" before you start doing other things:<br />
 - Rebuild Message Index<br />
 - Recount Board Totals<br />
 - Rebuild Members List<br />
 - Recount Membership<br />
 - Rebuild Members History<br />
 - Rebuild Notifications Files<br />
 - Clean Users Online Log<br />
 - Attachment Functions => Rebuild Attachments<br />
 Also double check Member Groups, and, if you have lists of blocked IPs from your old forum, go to "The Guardian" and copy those lists into their equivalent text boxes.~,
  

in %convlang_txt:

'finish' => qq~<p>We recommend you delete the file "$ENV{'SCRIPT_NAME'}". This is to prevent someone else running the converter and damaging your files. However, if you had 2 language encodings in your old forum: before deleting the Language Conversion files, check your new '$boardsdir/forum.control' file to make sure the board descriptions were correctly converted to UTF-8. If not, you can correct the error in Admin -&gt; Forum Controls -&gt; Boards -&gt; board to be edited OR you can hand edit '$boardsdir/forum.control' in a text editor such as Notepad++ by copying the description from your old forum's '$boardsdir/forum.control' into your new forum.control. Be sure to save your edited file with UTF-8 encoding.<br />
                 <br />
                 Further more, we strongly recommend to run the following "Maintenance Controls" in the "Admin Center" before you start doing other things:<br />
                 - Rebuild Message Index<br />
                 - Rebuild Members List<br />
                 - Recount Membership<br />
                 - Rebuild Members History<br />
                 - Rebuild Notifications Files<br />
                 - Clean Users Online Log<br />
                 - Attachment Functions => Rebuild Attachments<br /><br />
 Also double check Member Groups, and, if you have lists of blocked IPs from your old forum, go to "The Guardian" and copy those lists into their equivalent text boxes.<br />
                 <br />
                 <br />
                 You may now login to your forum. Enjoy using YaBB 2.7.00!</p>~,
  

In Convert.lng the intro is missing for 2x.
The intro is still displayed in English. After that, it continues to be German.

The language conversion is completely in English, although translated into German. Attention, some is not yet translated, since I must see the sentence constructions live. I attach the Convert.lng, can be that I have somewhere a mistake.

I can not find, but find this:

qq~$tabsep<span class="selected"><a href="$boardurl/ConvertLang.$yyext?$getlang" style="color: #f33; padding:0" class="selected">$tabfill UTF-8 Converter $tabfill</a></span>$tabsep&nbsp;~;
  

and this:

                <form action="$boardurl/ConvertLang.$yyext" method="post" style="display: inline;"> 

unfortunately no success

I did a complete run through Using German from Setup to Language conversion.
See the screen shots in the zip.

I don't have proper German copy of Setup.lng.  (pics 01.png - 08.png). So yeah, it's using Google translate so I can see that the script didn't call the English language file.

Pictures c01.png to cl11.png are using the lastest copy of Convert.lng that you posted.

One error I could see even though I don't read German.
Line 97:

'4mess' => qq~Um einen Timeout des Servers aufgrund der Anzahl der zu importierenden Messages zu verhindern, wird der Prozess in einen oder mehrere Schritte aufgeteilt.<br /><br />Der Zeitschritt (\$max_process_time) ist gesetzt auf <i>{max_process_time} Sekunden</i>.<br />Der letzte Schritt dauerte <i> {starttme} Sekunden</i>.<br />Die Konvertierung hat <i>{took} Minuten gedauert</i>.<br /><br /><i>$INFO{'total_threads'}</i> Messages wurden importiert.<br /><i>$INFO{'total_mess'}</i> Messages wurden importiert.<br />Es gibt <b>~ . ( $INFO{'totboard'} - $INFO{'count'} ) . qq~/$INFO{'totboard'}</b> &uuml;brige Boards, die mit Messages importiert werden.<br /><div style="float: left;">Es gibt <b>~ . ( $INFO{'totmess'} - $INFO{'tcount'} ) . qq~/$INFO{'totmess'}</b> &uuml;brige Threads zum Importieren. &nbsp; </div>~, 

should be something like:

'4mess' => qq~Um einen Timeout des Servers aufgrund der Anzahl der zu importierenden Messages zu verhindern, wird der Prozess in einen oder mehrere Schritte aufgeteilt.<br /><br />Der Zeitschritt (\$max_process_time) ist gesetzt auf <i>{max_process_time} Sekunden</i>.<br />Der letzte Schritt dauerte <i> {starttme} Sekunden</i>.<br />Die Konvertierung hat <i>{took} Minuten gedauert</i>.<br /><br /><i>$INFO{'total_threads'}</i> Threads wurden importiert.<br /><i>$INFO{'total_mess'}</i> Messages wurden importiert.<br />Es gibt <b>~ . ( $INFO{'totboard'} - $INFO{'count'} ) . qq~/$INFO{'totboard'}</b> &uuml;brige Boards, die mit Messages importiert werden.<br /><div style="float: left;">Es gibt <b>~ . ( $INFO{'totmess'} - $INFO{'tcount'} ) . qq~/$INFO{'totmess'}</b> &uuml;brige Threads zum Importieren. &nbsp; </div>~, 

The Menu items 'Members', 'Boards', 'Messages', 'Variables' probably need to be put into the language hashes.

The zip contains the current English Setup.lng and the bad German one I was using. (A lot of the German actually comes out of German/Admin.lng.)

'frmstart' => q~Das Startdatum des Forums wurde auf {forumstart} festgelegt, aber das erste Mitglied wurde {firstmember} registriert. Das Startdatum wurde deshalb auf {firstmember} abgeändert.~, 

Where can I paste this?
In the SVN it is not yet.

The error in answer 11 i corrected.

Tomorrow morning I make screeners of the installation.