The ULS method getFrequentLanguageList() leverages the getAutonym() method during the process to "make flat, make unique, and ignore unknown/unsupported languages" the result it returns - presumably trusting that getAutonym() returns the language code itself if the language "is unknown/unsupported".
Codes of 'redirected' languages, e.g. "fil" redirecting to 'tl', yield the autonym of the language it is redirected to when the getAutonym() method is called.
Consequently codes for redirected languages are not groomed by getFrequentLanguageList() - possibly resulting in return values containing e.g. both 'fil' and 'tl'.
Is this expected behavior?
This was found as part of the research for T217770