Hello xdiag !

I may suggest that this language isn't primary in your browser then by default UNA shows you another one by the first visit. The following code forms the language's choosing:

        if(!$sLang && !empty($_GET['lang']))

            $sLang = $this->tryToGetLang($_GET['lang'], $bSetCookie);

        if(!$sLang && !empty($_POST['lang']))

            $sLang = $this->tryToGetLang($_POST['lang'], $bSetCookie);

        if(!$sLang && !empty($_COOKIE['lang']))

            $sLang = $this->tryToGetLang($_COOKIE['lang']);

        if(!$sLang && !empty($_SERVER['HTTP_ACCEPT_LANGUAGE']))

            $sLang = $this->tryToGetLang($_SERVER['HTTP_ACCEPT_LANGUAGE']);

        if(!$sLang)

            $sLang = $this->tryToGetLang(getParam('lang_default'));

        if(!$sLang)

            $sLang = $this->tryToGetLang(BX_DOL_LANGUAGE_DEFAULT);

The bold lines show the taking language according to admin's setup.