Ja poprawiam obrys dziś/jutro wieczorem.

Ja bym poprawił zamiast usuwać.

"Officially in gmina Punsk there are two languages"

1. No. There is only one official language and one supplementary - only for contacts with local administration.
http://pl.wikipedia.org/wiki/Język_pomocniczy_(Polska)

2. "In some regions, places and streets have names in all local languages"
But those names are usually mapped to separate name:xx= tags. This is actual state in other multilang regions of Poland.

3. "People will also want to render maps in a specific language"
Right. Having separate tag for separate languages allows to render map in specific language. Glueing many languages in one tag, renders task much more difficult.

4. Can glued addresses be used in country-wide official documents?
Are used in company registers?
Personal documents?
Mail will be delivered?