This is working fine in 3.6.4. My feeling is that changes in the router breaks it.

Testing instructions

Create a default multilingual site at installation time, for example with 2 languages French and English.
Set English as default site language.
Set French as default site language for your super user.
Make sure Automatic Language Change is set to Yes in the languagefilter plugin.
Create login module form per language assigned to All pages

Create another article in each language.
Create a new single article menu item to display these new articles per language.
Associate these menu items or associate these articles (or both).
Use legacy router.

###How it is supposed to work
For the moment, do NOT choose a login redirect for the login module.
Also do not create a login menu item with a redirect.

Display the frontend English Home page.
===> When login with the login module, the super user should be redirected to the French Home page.
===> Result: the user says on the same page

Logout and now display the single article menu item in English.
===> When login with the login module, the super user should be redirected to the French associated article.
===> Result: the user stays on the same page.

In BOTH cases the user should be redirected to the French associated page (Home pages are ALWAYS associated de facto) : Home or single article menu item.

Logout.

Now choose in each login module a login redirect to their respective language single article menu item.

Display frontend Home page in English:
Login: ===> Result: the user says on the same page.
Display frontend Home page in French:
Login: ===> Result: the user says on the same page.

The same issue happens when using a login menu item and choosing or not a redirect.

Comments

The issue comes from a change in the value obtained by $this->app->getUserState('users.login.form.return') in https://github.com/joomla/joomla-cms/blob/staging/plugins/system/languagefilter/languagefilter.php#L631 in the onUserLogin() method.

In 3.6.4, we rightfully got for this variable something like index.php?Itemid=104&lang=fr-FR
Then with the regex $itemid = preg_replace('/\D+/', '', $this->app->getUserState('users.login.form.return'));

we got the $itemid, 104 in this case and the user is redirected to the correcty itemid in the correct language.

In 3.7.0 / staging, we get (SEF off):
http://localhost:8888/testinstall/trunkgitnew/index.php?option=com_content&view=article&id=6&Itemid=107&lang=fr
and therefore an $itemid value of 88886107 when we are not on the home page, or 8888 when on the home page instead of null

As can be seen in the onUserLogin() method, the possible case of a Login menu item using an URL and not the Itemid of a menu item is taken care before the check for elseif ($this->app->getUserState('users.login.form.return'))

This is an important regression for a multilingual site,

@Hackwar
@Bakual