An exception has been thrown during the rendering of a template ("Channel could not be found! Tip: You can use the Web Debug Toolbar to switch between channels in development.").
If you use the default theme, then the language select box will automatically appear in the top-left corner of your storefront as soon as you will have more than 1 locale available.
So, in the admin panel, create the locales you need in the Configuration -> Locales page and add them to the channel you need in the Configuration -> Channels page.