TYPO3 Core

If you set a default or override value for language in UserTSConfig like

setup {
    default {
        lang = de
    }

    // if set here we force that value
    override {
        lang = de
    }
}

it has no effect in the Backend. This is probalby due to code in
TYPO3\CMS\Setup\Controller\SetupModuleController::renderUserSetup
There a itemsProcFunc is called (renderLanguageSelect()), which just does not care about the above settings.
Seems the same in v8.

Same seems to be true for the setting startModule.
On both fields the label becomes gray (indicating disabled state) if set in override, but the select field itself is oblivious to the settings.