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It seems you're using custom language, and probably translation for JomSocial is not complete or corrupted. So could you check if this problem will also persist on default Joomla language?
This one does work: COM_COMMUNITY_INVITE_EMAIL_SUBJECT="%1$s nodigt je uit om lid te worden van %2$s"
This one doesn't: COM_COMMUNITY_NOTI_ADD_YOU_AS_FRIEND="%1$s wil je vriend worden"
I'm very sorry but debuging custom translations is beyond our support scope, could you maybe consult about this problem with team which made this language for JS ?