Machine Translation Systems may have all source and target languages enabled or only certain ones. All are enabled by default; however, this may be changed (see Modifying an Existing Translation System).
If you want to simply see the languages that are currently enabled for a specific machine translation system, then click on View Languages.
This will open a popover, which shows currently enabled source and target languages for the selected machine translation system. To close this window, simply click on another area of the screen. In this example only one source language (English) and one target language (French) have been enabled.
If all languages are currently enabled, the popover will appear as shown below when languages are viewed:
