Please edit the file administrator/components/com_gantry5/templates/partials/layout.html.twig and find the line:
<script type="text/javascript" defer="defer" src="{{ url('gantry-admin://assets/js/main.js') }}"></script>
Right after it add a new line with the following:
{% include "@gantry-admin/partials/js-translations.html.twig" %}
This should fix the issue for you. Alternatively, you can download and install the latest CI Build from
gantry.org/downloads#ci-builds
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