Steps to reproduce the issue

• Install fresh Joomla 3.6
• Install Sample Data = No
• Extra step: Install 1 language (in my case 'German DE')
• Activate the multilingual feature = Yes
• Install localised content = Yes
• Enable the language code plugin = No
• Default Administrator language = German
• Default Site language = German

• Finish install

• Open BE

• Global Configuration - SEO Settings: -- Search Engine Friendly URLs = Yes -- Use URL Rewriting = Yes -- Adds Suffix to URL = Yes
• Enable .htaccess

• Edit Plugin: System - Language Filter - set 'Remove URL Language Code' = Yes

• Call FE

• in my case 'German' site is open
• switch to EN - works fine
• switch to DE - works fine
• switch to EN - works fine
• switch to DE - doesn't work!

Expected result

• switching the language on start-items should work all the time

Actual result

• switching the language on start-items doesn't work after 4th klick to switch

System information (as much as possible)

• multiple host / multiple PHP (5.6 & 7)
• multiple MySQL and MySQLi

Additional comments

Site still stays in EN, even if the URL is cleaned up and cache is reset.
I only can recall the DE-site (default language) when I quit the browser (ending cookie?).
Could it be a cookie-thing?
This occours most likely since update to 3.6.

!!! Courios: This occurs on MAC on FireFox, GoogleChrome, on PC as well on FireFox and IE,
but NOT on MAC Safari .... - On MAC Safari it is still working as it should do.

Workaround

Set within the Plugin 'System - Language Filter' -> 'Remove URL Language Code' = No