props: {
  maxLength:  { type: Number, default: 20 },
}

i18n: {messages: {
          en: { name_length: "Max. {this.maxLength} characters" },...

}

"Max." + String(this.maxLength) + characters", but it comes out as undefined.

v-model="text"      :prepend-icon="iconfront"
                :rules="nameRules"  :name="name"
                :label="label"      :type="type">

         </v-text-field>

</template>

<script>

     export default {

            props: {
                value: {type: String},
                iconfront:  { type: String },
                name:       { type: String },
                label:      { type: String },
                type:       { type: String, default: 'text' },
                minLength:  { type: Number, default: 1 },
                maxLength:  { type: Number, default: 20 },
            },
            computed: {
                text: { get() { return this.value },
                        set(val) { this.$emit('input', val) }
                }
            },
            data () {
                return {
                    nameRules: [

                    (v) => !!v || this.$i18n.t("name_rule"),
                    (v) => v && v.length <= this.maxLength || this.$i18n.t("name_length")

                    ]
                }
            },
            methods: {
                onInput(input){
                     this.$emit('textFieldInput', input)
                }
            },
            i18n: {
                messages: {
                    en: {
                            name_rule: "required field",
                            **name_length: "Max. {this.maxLength} characters",**
                            confirmation_rule: "passwords must match",
                            email_rule: "email must be valid",
                            password_length: "Length must be" + String(this.minLength)+ "-" + String(this.minLength) + "characters",
                    },
                    de: {
                            name_rule: "Pflichtfeld",
                            name_length: "Max. 20 Zeichen",
                            confirmation_rule: "Passwörter müssen übereinstimmen",
                            email_rule: "Email muss gültig sein",
                            password_length: "Länge: 6-20 Zeichen",
                    },
                    fr: {
                            name_rule: "champs requis",
                            name_length: "20 caractères maximum",
                            confirmation_rule: "les mots de passe doivent correspondre",
                            email_rule: "email doit être valide",
                            password_length: "longueur requise: 6 à 20 caractères",

                    },
        }
            },     //end of translations
     }


</script>