BootstrapVueのNavbarが上手く表示されません

import Vue from 'vue'
import App from './App.vue'
import { BootstrapVue, BootstrapVueIcons } from 'bootstrap-vue'

import 'bootstrap/dist/css/bootstrap.css'
import 'bootstrap-vue/dist/bootstrap-vue.css'

Vue.use(BootstrapVue)
Vue.use(BootstrapVueIcons)

Vue.config.productionTip = false

new Vue({
  render: h => h(App),
}).$mount('#app')

<template>
  <div id="app">
    <navbar></navbar>
  </div>
</template>

<script>
import navbar from './components/navbar.vue'

export default {
  name: 'App',
  components: {
    navbar
  }
}
</script>

<style>
#app {
  font-family: 'Avenir', Helvetica, Arial, sans-serif;
  -webkit-font-smoothing: antialiased;
  -moz-osx-font-smoothing: grayscale;
  text-align: center;
  color: #2c3e50;
  margin-top: 60px;
}
</style>

<template>
<div>
  <b-navbar toggleable="lg" type="dark" variant="info">
    <b-navbar-brand href="#">NavBar</b-navbar-brand>

    <b-navbar-toggle target="nav-collapse"></b-navbar-toggle>

    <b-collapse id="nav-collapse" is-nav>
      <b-navbar-nav>
        <b-nav-item href="#">Link</b-nav-item>
        <b-nav-item href="#" disabled>Disabled</b-nav-item>
      </b-navbar-nav>

      <!-- Right aligned nav items -->
      <b-navbar-nav class="ml-auto">
        <b-nav-form>
          <b-form-input size="sm" class="mr-sm-2" placeholder="Search"></b-form-input>
          <b-button size="sm" class="my-2 my-sm-0" type="submit">Search</b-button>
        </b-nav-form>

        <b-nav-item-dropdown text="Lang" right>
          <b-dropdown-item href="#">EN</b-dropdown-item>
          <b-dropdown-item href="#">ES</b-dropdown-item>
          <b-dropdown-item href="#">RU</b-dropdown-item>
          <b-dropdown-item href="#">FA</b-dropdown-item>
        </b-nav-item-dropdown>

        <b-nav-item-dropdown right>
          <!-- Using 'button-content' slot -->
          <template #button-content>
            <em>User</em>
          </template>
          <b-dropdown-item href="#">Profile</b-dropdown-item>
          <b-dropdown-item href="#">Sign Out</b-dropdown-item>
        </b-nav-item-dropdown>
      </b-navbar-nav>
    </b-collapse>
  </b-navbar>
</div>
</template>

<script>
export default {
  name:'navbar'
}
</script>

<!-- Add "scoped" attribute to limit CSS to this component only -->
<style scoped>

</style>