my test method:

"First, make sure you set the menu type (name) of the default language is 'mainmenu' (or any name) and other menus should be name in format 'mainmenu_fr' (fr is the language code, you should replace it with 2 character of corresponding language).
Second, make sure you have set default page (should be the home menu item) for each menu (each language). I suppose you have did it.
Next, just open the megamenu configuration and save the menu configuration (even you do not change anything).

Select the 'mainmenu' in Menu dropdown of Navigation tab in Template Manager.

The second solution is that, you need to duplicate the template style. Assign each style to corresponding language.
Then each template style should target to correct menu in Menu dropdown."

Hi,

Please try to upgrade to T3 1.4.3. Then change mainmenu_fr to mainmenu-fr and save the configuration again.

I am sorry, there may be a typing error.

Regards