choroba, while substr might appear easier, it is not shorter. Also, I don't use substr enough to remember it is in the toolbox.

As for the Croatian and Czech month name position problem and if I were to have to choose between them, I would choose to add Czech since it has almost twice the amount of speakers then Croatian. I can't think of an easy way to fix the problem. I could add something where the user is given the option to add problematic languages if needed.

get_month_number('listopad', {'Czech or Croatian' => 'Czech'});
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Other than that, I'm not sure.