Add human languages information to metadata

Users tend to prefer Apps in languages they understand well, but in F-Droid they can't know in advance whether an APK is available in certain language. It would be unrealistic to assume that the final user of a global project understands English well, or at all.

Automatic detection of languages is possible even for the majority of Apps already present in the repo, the presence of value-xx folders in the source code, where xx is the language code, indicate that the App is translated, either fully or to a point that satisfied the developers.

EXCEPTIONS

• English could be assumed as this is the de-facto standard. The presence of values-en should raise an exception.

• These value-xx folders are usually located under app/src/main/res/ but there are exceptions.

For cases where automatic detection is not possible another code (let's say '00') could be used to indicate that the full list of languages is unknown.

Metadata could be perfected manually either before adding the App to the repository or at a later stage.

USE CASES

• F-droid client and website could present the information to the user and offer filtering by language(s).
• Translation teams could have interest in gathering this information directly from metadata and treat it in other ways.

PRE-REQUISITE

To include languages information to metadata, a modification to the XML structure might be necessary.