set language default in TwigTransTest better using the constructor or something

Follow-up to #2337869: set language weight in EntityTranslationTest better using the constructor or something

Problem/Motivation

#2226533: Changes to the Language class due to the LanguageInterface (followup) was

diff --git a/core/modules/system/src/Tests/Theme/TwigTransTest.php b/core/modules/system/src/Tests/Theme/TwigTransTest.php
index 10adc3b..7f71619 100644
--- a/core/modules/system/src/Tests/Theme/TwigTransTest.php
+++ b/core/modules/system/src/Tests/Theme/TwigTransTest.php
@@ -70,12 +70,12 @@ protected function setUp() {

// Assign Lolspeak (xx) to be the default language.
$language = \Drupal::languageManager()->getLanguage('xx');
- $language->default = TRUE;
+ $language->setDefault(TRUE);
language_save($language);
$this->rebuildContainer();

// Check that lolspeak is the default language for the site.

but #2334763: Tidy up of LanguageInterface - removal of setters and other unnecessary methods is taking out setDefault()

Proposed resolution

Remaining tasks

User interface changes

No.

API changes

No.