in Fruity TwentyThree theme only one language is displayed (although all languages are loaded in html ul).
we figured out, that views/subviews/navigation/language_changer_top_menu.twig has to deal with it and made a workaround:

{# There is many condition where the language changer should not be shown. Eg: there is only one language available #}
{% if aSurveyInfo.alanguageChanger.show == true %}

{# we set a shortcut for aSurveyInfo.alanguageChangerDatas #}
{% set aLCD  = aSurveyInfo.alanguageChanger.datas %}

<!-- Language Changer, top menu version -->
<li class="dropdown-header text-uppercase">{{ gT("Select language") }}</li>
{% for value, lang in aLCD.aListLang %}
    <li class="index-item">
        <a href='#' data-limesurvey-lang='{{ value }}' class="dropdown-item ls-language-link">
            {{ lang }}
        </a>
    </li>
{% endfor %}

{# <li class="ls-no-js-hidden form-change-lang dropdown nav-item">#}
{# <a class="nav-link dropdown-toggle" role="button" data-bs-toggle="dropdownCustom"#}
{# data-bs-target="#language-dropdown" data-navtargetid="language-dropdown">#}
{# #}{# NOTE: {{gT( aLCD.aListLang[aLCD.sSelected] )}}: retreives the language name of the selected language and translates it #}
{# {{ aLCD.aListLang[aLCD.sSelected] }}#}

{# </a>#}
{# <ul class="dropdown-menu dropdown-menu-end" id="language-dropdown">#}
{# {{ include('./subviews/navigation/back.twig') }}#}
{# #}{# List of available languages. #}
{# {% for value, lang in aLCD.aListLang %}#}
{# <li class="index-item">#}
{# <a href='#' data-limesurvey-lang='{{ value }}' class="dropdown-item ls-language-link">#}
{# {{ lang }}#}
{# </a>#}
{# #}
{# {% endfor %}#}
{# #}
{# #}
{% endif %}

{{ registerScript("BasicThemeScripts-TopLanguageChanger", "basicThemeScripts.initTopMenuLanguageChanger('.ls-language-link ', 'form#limesurvey'); ", "POS_POSTSCRIPT") }}