i18n files

## Describe the enhancement

We'd like to see a count of our localized text files on the repository's language breakdown. Is there a way to achieve this?

Our automated processes store translated strings in *.properties files in our project (e.g. i18n/checkout_de_DE.properties).

Because of the definitions and file extension mappings in languages.yml, these get classified as INI files; but `type: data` means they are not shown on the repository's languages breakdown.

Using .gitattributes we can map the file paths to languages defined in languages.yml, but no language seems suitable to describe translated text. And AFAICT there is currently no way for us to define a custom "I18n" entry in order to map file paths to it (via .gitattributes) so that those counts can be displayed on the repository's main page.

Start by reading languages.yml and the .gitattributes language-mapping behavior described in the issue. Determine how a repository could represent translated .properties files in the language breakdown, and consider the requested custom I18n entry; done means those files can be counted there without misclassifying their content.

Written by the indexing model from the issue text.