As a meteorologist, these translations have bothered me as well - however, the article of the word Graupel was never my main concern. :-)
It is the World Meteorological Organization that regulates how weather should be observed. One of the world-wide most common ways of reporting weather and sending observations for distribution all over the world is by using the "synop code". (The official English version of the synop code can be found at http://www.wmo.int/pages/prog/www/WMOCodes/Ma...
; precipitation is covered on pages 64-66 of this document.)
Of course, there is a German translation of the synop code as well (www.deutscher-wetterdienst.de/lexikon/download.php?file=Bodenwetterkartensymbole.pdf; the precipitation part can be found on pages 5-6).
By comparing these two documents, we find (among other things) that
ice pellets = Eiskörner
snow pellets = Graupel
(There is no "sleet" in the official synop code.)
The American Meteorological Society (http://amsglossary.allenpress.com/glossary/se...
sleet - 1. See ice pellets. 2. In British terminology, and colloquially in some parts of the United States, precipitation in the form of a mixture of rain and snow.
ice pellets - A type of precipitation consisting of transparent or translucent pellets of ice, 5 mm or less in diameter. They may be spherical, irregular, or (rarely) conical in shape. Ice pellets usually bounce when hitting hard ground and make a sound upon impact.
snow pellets - (Also called soft hail, graupel, tapioca snow.) Precipitation consisting of white, opaque, approximately round (sometimes conical) ice particles having a snowlike structure, and about 2–5 mm in diameter.
The UK Met Office mentions ice pellets and snow pellets, with identical or very similar descriptions, in their "Fact Sheet 17, Weather Observations" (http://www.metoffice.gov.uk/corporate/library...
Deutscher Wetterdienst (http://www.dwd.de/bvbw/appmanager/bvbw/dwdwww...
Schneeregen - Bezeichnung für gleichzeitiges Fallen von Schnee und Regen in Form von Schauern oder auch als Dauerniederschlag.
I suggest that the entries
sleet [meteo.] der Graupel
ice pellet [meteo.] der Graupel
are deleted (the English terms may be common, but the translation is incorrect), and that the entries
snow pellet [meteo.] der Graupel
soft hail [meteo.] der Graupel
If, as some of the previous contributors have stated, the terms snow pellets and soft hail are not very widely known, maybe there's a way of indicating that?