fix: translation only applied on app start

[mesemus]

Description

This pull request fixes premature evaluation of i18n strings inside static fields. gettext call, that causes
the evaluation to be performed at boot time, was replaced with gettext_lazy that is performed at
request time. This fixes Invenio string not translated when language changes (though it is not expected
to change).

Checklist

Ticks in all boxes and 🟢 on all GitHub actions status checks are required to merge:

• I'm aware of the code of conduct.
• I've created logical separate commits and followed the commit message format.
• I've added relevant test cases.
• I've added relevant documentation.
• I've marked translation strings.
• I've identified the copyright holder(s) and updated copyright headers for touched files (>15 lines contributions).
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[mesemus]

Please backport this to RDM12 as well.