You would have to add a skins/default/xui/gr/ folder with the *.xml translation files for Greek. I don't think there are any Greek translations for any viewer for now, but you can still have a look at another language folder in Snowglobe (Snowglobe-1.4.2/skins/default/xui/de/ for example).
Each of those XML files correspond to a file in the skins/default/xui/en-us/ folder (which is the only folder to hold the full floaters/panels/menus definitions): for each element in the English (en-us) version of the floater/panel/menus definition, you must add a similar definition in your translation XML file, but only specifying the UI element name (name="" parameter, which must be the same as in the English menu), and the translated label and/or text for that element. The viewer will then override the label/text for each UI element in the en-us XML file with the one defined in the corresponding language translation XML file, based on the UI element name.
Example, in a menu definition, you could have the following button:
and then in the German translation file, you would have:
Be aware that when translating, it is best keeping the translated text length equal to or shorter than the English text, since the room for that text in the floater is adjusted for the English text and using a larger text might result in truncated or overlapping text elements... You can test the translated floaters with the Advanced -> XUI -> Load from XML feature (which alas can't help in the case of panels and menus).