Wikipedia:WikiProject Geographical coordinates/region:

The region: parameter specifies the political region for terrestrial coordinates. It is used to select appropriate map resources. If no region: parameter is provided, GeoHack attempts to determine the region from the coordinates.

The region should be supplied as either a two character ISO 3166-1 alpha-2 country code or an ISO 3166-2 region code.

Examples of ISO 3166-1 alpha-2 codes:

AQ Antarctica
AU Australia
BR Brazil
DE Germany
GB United Kingdom
HK Hong Kong
IN India
LK Sri Lanka
RU Russia
US United States

Examples of ISO 3166-2 region codes:

DE-TH Thuringia, Germany
GB-BIR Birmingham, England
NO-03 Oslo, Norway
US-NY New York state, US

The oceans have the following Wiki assigned code elements per de:Vorlage:Coordinate#Ozeane.

XN Arctic Ocean
XA Atlantic Ocean
XI Indian Ocean
XP Pacific Ocean
XS Southern Ocean

In addition, two Wiki assigned code elements can be used with {{coord}}:

XZ for objects in or above international waters (similar to UN/LOCODE).
ZZ for use in examples.

Samples
Focus region	Region	Markup	Result
Switzerland	`CH`	`{{coord\|46.9524\|N\|7.4396\|E\|region:CH}}`	46°57′09″N 7°26′23″E / 46.9524°N 7.4396°E / 46.9524; 7.4396
Berlin, Germany	`DE-BE`	`{{coord\|52.5164\|N\|13.3775\|E\|region:DE-BE}}`	52°30′59″N 13°22′39″E / 52.5164°N 13.3775°E / 52.5164; 13.3775