The game treats this as two concepts, albeit linked. "FIFA Nationality" is surely "Nations Eligible For". The below player retains his Spanish nationality but is no longer eligible for Spain due to having represented Uruguay. Hypothetically, If he goes on to play in the USA and gains U.S. nationality, he does not gain a new nation to be eligible for, but he will gain a new Other Nationality. Or are they still just "FIFA Nationalities" in your opinion?
Also, playing devil's advocate, why link nationality, eligibility, and language so tightly for managers? I may be wrong but I know of no reason to do so in the game. A manager can manage anywhere. There is no "FIFA Nationality" for management only national rules governing employment, which I don't believe are in the game (for managers). Plus, being a "FIFA National" is no guarantee of speaking the language.
Either way, if we continue playing devil's advocate and treat the Nationality field as "FIFA Nationality", I still can not create "me" in the game, as a Hannover-born, English Nationality, who can not speak German. There is no workaround for this because of how the above has been implemented. Something small that I would happily see changed and hence the feature request.