Quasar + Vue plugin issue

// component.vue
export default {
    ...
    methods: {
        myMethod: function() {
            this.$axios.get('/call', {
            }).then((resp) => {
                this.$resp.processResponse(resp); // here I'm using my plugin ($resp)
            }, (error) => {
                console.log(error);
            });
        }
    }
}

// plugins.js, loaded by Quasar boot, seems OK
import Vue from 'vue';
import resp from './plugins/responses.js';

const plugin = {
    // eslint-disable-next-line no-shadow
    install (Vue) {
        Vue.prototype.$resp = resp; // this installs plugin, seems OK
    }
};
Vue.use(plugin);


// responses.js
// this is my plugin
import Vue from 'vue';

export default {
    processResponse() {
        const text = this.$t('label'); // throws error (see below)
        const text2 = Vue.prototype.$t('label'); // throws error (see below)
    }
};

// inside Vue!
Vue.prototype.$t = function (key) {
    var values = [], len = arguments.length - 1;
    while ( len-- > 0 ) values[ len ] = arguments[ len + 1 ];

    var i18n = this.$i18n;

    // here fails with: "vue-i18n.esm.js?a925:182 Uncaught (in promise) TypeError: Cannot read property '_t' of undefined"
    return i18n._t.apply(i18n, [ key, i18n.locale, i18n._getMessages(), this ].concat( values ))
};