Read coordinates with variable Gauß-Krüger zone (meridian) in QGIS

I know no way to import point data with multiple projections from a single txt file into QGIS at once.

My quick-hack-solution would be to sort by x-coordinate, save all points within a single Gauß-Krüger zone as a separate txt-file, then load all 3 (?) of them as delimited text files with their respective correct coordinate systems (make sure on-the fly projection is activated).

Your project's projection will depend on your base map, but now all cities should show up in the correct places. I think you will even be able to merge them into a single point file.

The short answer is yes.

A good strategy without losing the data integrity consists into representing all the points in the same SRS, i.e. WGS84. In order to do this, you can:

1. Add hauptstadt.txt as CSV in QGIS using custom delimiter as space, encoding ISO-8859-1, trim fields, skip empty fields and geometry type none.
2. Save hauptstadt as DBF (i.e. hauptstadt.dbf) and load it in the ToC.

3. Open the attribute table, start editing and calculate a new integer field SRID as:

    CASE
      WHEN regexp_match("x",'^(2[0-9]{6})$')=1 THEN 31466  
      WHEN regexp_match("x",'^(3[0-9]{6})$')=1 THEN 31467  
      WHEN regexp_match("x",'^(4[0-9]{6})$')=1 THEN 31468  
      WHEN regexp_match("x",'^(5[0-9]{6})$')=1 THEN 31469
    END
   

4. Open a shell, change directory to the folder containing hauptstadt.dbf and then execute:

   ogr2ogr hauptstadt_4326.shp hauptstadt.dbf -dialect SQLite -sql "SELECT *, ST_Transform(MakePoint(x, y, SRID), 4326) AS geometry FROM hauptstadt"
   

   ogr2ogr will return hauptstadt_4326.shp and ... you're done!