First decide what the software answer to your users will be.
Will you beep on digit press and not accept it ?
Will you let the user type everything then beep at him ?
Will you silently swallow anything not being A-Z; a-z ?
If you want to be harsh, just set Validated by Calculation, with this formula:
Filter(Lower(Self); "qaywsxedcrfvtgbzhnujmiklopöüä-'èéàùçñ") = Lower(Self)
and a custom display message if validation fails, like "Would you be proud to have a name like this ?"
isempty ( filter ( self ; "1234567890" ) )
Would that work?
if you want to allow "ƒɨŁēṃǟǩęř Ȑůɫž", yes.
Sorry for not being specific.
I'm using file maker pro 14, I would just simply like to show the message if validation fails.
So the calculation returns 1 if the string is incorrect?
Given the variety of characters used around the world, yes, I would. As DavidJondreau points out, even disallowing numbers could keep someone from entering a perfectly valid name.
Of course, if someone's habitually entering numbers in a name field, or "ƒɨŁēṃǟǩęř Ȑůɫž", I might advocate putting an audit trail on the name field to see what user is in need of some training in basic data entry skills.