Resolved
Resolved in: WPML 4.6.9
Overview of the issue
When you have a static homepage set up in WordPress and your language URL format is set to Different languages in directories in WPML → Languages, adding a language parameter (e.g. /lang=en) while visiting your home page displays the blog page and not the static home page that you’ve set.
This is a known issue, and it is happening because it’s also possible to use WPML to format your URLs with the language name added as a parameter.
Workaround
You can configure a 301 redirect from the language parameter to the new URL using the Redirection WordPress plugin.