{% set sub = [
    {
        icon: 'fa:flag',
        label: __('Messages'),
        link: path('translation', {'domain': 'messages'}),
        isallowed: 'translation'
    }, {
        icon: 'fa:flag',
        label: __('Long messages'),
        link: path('translation', {'domain': 'infos'}),
        isallowed: 'translation'
    }, {
        icon: 'fa:flag',
        label: __('Contenttypes'),
        link: path('translation', {'domain': 'contenttypes'}),
        isallowed: 'translation'
    }
] %}

{{ nav.submenu('fa:flag', __('Translations'), sub, (page_nav == 'Settings/Translations')) }}

{% set tr_menu = app['menu.admin'].child('translations') %}
{% set sub = [
    {
        icon: tr_menu.child('tr_messages').icon,
        label: tr_menu.child('tr_messages').label,
        link: tr_menu.child('tr_messages').uri,
        isallowed: tr_menu.child('tr_messages').permission
    }, {
        icon: tr_menu.child('tr_long_messages').icon,
        label: tr_menu.child('tr_long_messages').label,
        link: tr_menu.child('tr_long_messages').uri,
        isallowed: tr_menu.child('tr_long_messages').permission
    }, {
        icon: tr_menu.child('tr_contenttypes').icon,
        label: tr_menu.child('tr_contenttypes').label,
        link: tr_menu.child('tr_contenttypes').uri,
        isallowed: tr_menu.child('tr_contenttypes').permission
    }
] %}

{{ nav.submenu('fa:flag', __('Translations'), sub, (page_nav == 'Settings/Translations')) }}

Implement menu object in translations menu Twig template
Implement menu object in translations menu Twig template