i18n: use gettext for translatable strings in admin

* Replaced the custom _ function with gettext from invenio_i18n to standardize internationalization and make translatable strings compatible with Invenio's i18n

invenio_oaiserver/admin.py

@@ -2,22 +2,19 @@
 #
 # This file is part of Invenio.
 # Copyright (C) 2015-2018 CERN.
+# Copyright (C) 2024 KTH Royal Institute of Technology.
 #
 # Invenio is free software; you can redistribute it and/or modify it
 # under the terms of the MIT License; see LICENSE file for more details.
 
 """Admin model views for OAI sets."""
 
 from flask_admin.contrib.sqla import ModelView
+from invenio_i18n import gettext as _
 
 from .models import OAISet
 
 
-def _(x):
-    """Identity."""
-    return x
-
-
 class OAISetModelView(ModelView):
     """OAISets model view."""