Drupal 8 TMGMT Language Picker
The Drupal 8 language picker can be set up by going to the Structures > Blocks page. It's not from theTMGMT module, but from Drupal 8 core. There's a list of a regions where you can put blocks.
Select the region and click Place Block. You'll then be able to choose a Language switcher block in a pop up list.