Several developers recently have been confused by certification failures for 6.13.8 You must localize your app for all languages that it supports . These failures complain that the app doesn’t support a language that the developer had no intention of supporting. Checking the dashboard shows no mention the language anywhere there (what the certification failure is about), checking the manifest finds no reference there, and checking the app’s resources finds no mention there either.
So where is this unexpected language reference hiding? Look in the smallest directory names in the appx project.
Windows Store apps store localized resources in directories named by the BCP-47 language tag (See How to name resources using qualifiers), which generally look something like “en-us” or “fr-fr”, but which can be shortened to include just the initial language code (e.g. “en” or “fr”). This means that if you have a folder which unintentionally matches a language code it will be misinterpreted as containing that language’s resources and certification will expect the app to fully support that language.
This often comes up with languages like “Bulgarian” whose ISO-631 code “bg” looks like an abbreviation for a common term such as “background”.
To fix the problem, you'll need to rename the directory to a longer and more unique name.
Follow the Windows Store Developer Solutions team @wsdevsol. Comments are welcome, both below and on twitter.