v:page.languageMenu should use Site configuration in TYPO3 9/10

[yol]

Since TYPO3 9, setting up languages for frontend usage is handled in site configuration. LanguageMenuViewHelper does not consider this data, but only uses the sys_language DB table. It would be convenient if it used the site configuration instead, since more information is available there (such as the name of the default language).

[yol]

For more info, see https://docs.typo3.org/m/typo3/reference-coreapi/10.4/en-us/ApiOverview/SiteHandling/AddLanguages.html and https://api.typo3.org/10.4/class_t_y_p_o3_1_1_c_m_s_1_1_core_1_1_site_1_1_entity_1_1_site_language.html