docs: Update i18n.md

Author: aklinker1

docs/guide/essentials/i18n.md

@@ -81,6 +81,10 @@ However, it is recommended you stick with the vanilla API (or a package based on
 - Translations are not bundled multiple times, keeping your extension small
 - Zero configuration
 
+However, there is one major downside to the vanilla API and any packages built on top of it:
+
+- Language cannot be changed without changing your browser/system language
+
 Here are some examples of how to setup a third party i18n library:
 
 - [vue-i18n](https://github.com/wxt-dev/wxt-examples/tree/main/examples/vue-i18n)