Moment local with locales it doesn't work when I change the language

[Teodor]

Well the moment framework options are available for datepicker component only.
You need a custom formatter as explained here by Patrick in order to get it working with your date formatting on the page ... :

How to get date with st nd or th using formatdate

Using PHP won’t work here as PHP is parsed before the page is send to client side. And all the Wappler magic happens on client side. So next_birthday does not exist as far as PHP is concerned. Only options I see here is to either wait for a solution from Wappler… or manually put in conditions for placing st, nd or th after the date’s date part.