Locale: when entering page, guess language.

If user picks an installer language on the welcome page, then
the system language follows that setting -- but only after the
first action where Calamares must guess the system language
based on location. So guess on entry, so that system language
follows installer language.

FIXES #688.

src/modules/locale/LocalePage.cpp

@@ -422,6 +422,13 @@ void
 LocalePage::onActivate()
 {
     m_regionCombo->setFocus();
+    if ( m_selectedLocaleConfiguration.isEmpty() ||
+         !m_selectedLocaleConfiguration.explicit_lang )
+    {
+        auto newLocale = guessLocaleConfiguration();
+        m_selectedLocaleConfiguration.lang = newLocale.lang;
+        updateLocaleLabels();
+    }
 }