Drupal 7 introduced a new pluggable language negotiation system but the default setup was empty. If you enabled language support, you could add as many languages you wanted but the languages were not accessible. If you enabled the language switcher block, it did not actually appear. That was all due to the empty default configuration. Drupal 8 comes with a better default, so if you enable language support, it now assumes you wanted to have path prefix language negotiation by default. This makes it much easier to start up with a new site and is still as easily configurable to different settings if desired.
The change affects module developers writing tests involving language support. Drupal 7 tests should have assumed that there was no language negotiation turned on, so paths were generated in their original form, etc. Drupal 8 tests can/should assume path language negotiation is turned on. This does not affect test that do not work with the language/locale modules.