docs/.vitepress/theme/index.ts

import { h } from 'vue'
import DefaultTheme from 'vitepress/theme'
import LanguageSwitcher from './LanguageSwitcher.vue'
import './custom.css'

export default {
    extends: DefaultTheme,
    Layout: () => {
        return h(DefaultTheme.Layout, null, {
            'nav-bar-content-after': () => h(LanguageSwitcher),
        })
    },
    enhanceApp({ app }) {
        app.component('LanguageSwitcher', LanguageSwitcher)
    }
}