i18n.md

If you have access to bash osx/Linux: cd inside your grails application folder (its root) and run:

grep -r message *|grep "code"|grep "default"|awk '{
        if( match($0, /code:.*'[A-z]+'/) ) {
                str1=substr($0, RSTART, RLENGTH);
                code=substr(str1, index(str1,":"));
                code1=substr(code,  index(code,":")+index(code,"'\''"),index(code,",")-(index(code,"'\''")+2));
                print code1;
        }
        if( match($0, /code=.*"[A-z]+"/) ) {
                str1=substr($0, RSTART, RLENGTH);
                code=substr(str1, index(str1,"="));
                code1=substr(code,  index(code,"=")+index(code,"\""),index(code," ")-(index(code,"\"")+2));
                print code1;
        }

   }'|sort|uniq 

On mine i got:

default.add.label
default.Authors.deleted.label
default.authors.label
default.authors}.label
default.Authors.missing.label
default.Authors.notfound.label
default.Books.deleted.label
default.books.label
default.books}.label
default.Books.missing.label
default.Books.notfound.label
default.button.create.label
default.button.delete.label
default.button.edit.label
default.button.update.label
default.created.message
default.deleted.message
default.delete.label
default.details.updated.label
default.id.missing.label
default.lang.changed.label
default.list.label
default.logout.success.label
default.not.found.message
default.Numbers.deleted.label
default.numbers.label
default.numbers}.label
default.Numbers.missing.label
default.Numbers.notfound.label
default.own.username.label
default.password.missing.label
default.password.update
default.save.label
default.token.missing.label
default.token.notfound.label
default.updated.message
default.update.label
default.username.available.label
default.username.deleted.label
default.username.missing.label
default.usernamepassword.invalid.label
default.username.update
default.username.used.label
default.user.notfound.label
${domainClass.propertyName}.label
${domainClass.propertyName}.${p.name}.label
language.de
language.en
security.newusername.label
security.password.confirm.label
security.password.label
security.register.label
security.signin.label
security.signoff.label
security.username.label