Tiny Java library to roughly guess country from latitude/longitude

Question

I develop an Android app that lets people upload a picture and tag it. When the picture's EXIF data has GPS coordinates in Japan, I would like to add #japan to the tag suggestions.

Requirements:

• 100% Java library
• Input: latitude, longitude
• Output: Country name in English (even better if other languages are available)
• Size: Below 2 MB, as it is not a central feature and I want to keep the app small (the whole app is currently 2MB)
• Precision: About 100 kilometers.
• Response time: Less than half second even on a cheap phone
• Must work offline
• Free and open source

Tolerance:

• In border areas, giving several countries is OK, for instance for a picture taken in Luxembourg (tiny country) it could suggest France,Germany,Luxembourg,Belgium. Giving only one suggestion is OK too.
• Out-of-date borders is not a big problem. No update mechanism is needed as the app itself gets updated quite regularly.
• Behavior at sea is not important, so using one big rectangle (or circle) for New Zealand is OK, for instance.

Keyword: reverse geocoding

Answer 1

6 years later, still no such library, so I just created one and released it as open source:

https://github.com/coordinates2country/coordinates2country

• Works offline
• Response time: Less than 100 milliseconds
• Size: 100 kilobytes
• Does not hoard any RAM
• Precision is about 0.2 degrees

Example usage:

Coordinates2Country.country(50.1, 10.2) returns "Germany".