Revert translations done by a specific user on a specific day

Hello, I work on JOSM, and the user who added most of a translation to Launchpad is telling us that (a) it wasn't him and (b) the translations are wrong. The poor translations from him were done over a single day. Is there a way for us to revert those translations? (Note: This was done in 2015).

Our ticket, for those who want to read it: https://josm.openstreetmap.de/ticket/21720

The user (mxn) has told us that on 2015-05-12, someone/something uploaded around 7800 strings using his account.
Filter: https://translations.launchpad.net/josm/trunk/+pots/josm/vi/+filter?person=mxn

We would like to do the following:
1. Reset/revert strings changed by mxn on 2015-05-12 for the Vietnamese translation.

What I have tried:
1. Writing a script to reset the translations (or set them to "")
Partial script for (1) in `What I have tried` (partial since it only gets the VI translation):
```
#!/usr/bin/env python
# -*- coding: utf-8 -*-

from typing import Optional
from launchpadlib.launchpad import Launchpad, ProjectSet
from launchpadlib.credentials import Consumer
from lazr.restfulclient.resource import Entry

def main():
    cachedir = './cache'

    launchpad: Launchpad = Launchpad.login_with(
        'josm ticket #21720: reset vi translation', 'production',
        cachedir, version='devel')
    josm: Entry = launchpad.projects("JOSM")
    trunk: Entry = josm.series[0]
    template: Entry = trunk.getTranslationTemplates()[0]
    translation: Optional[Entry] = None
    for tTrans in template.translation_files:
        if "josm/vi" in tTrans.self_link:
            translation = tTrans
            break
    if not translation:
        raise NotImplementedError("Don't currently know what to do if translation isn't found")
    # At this point, nothing else can be done -- there are no further API calls to make

if __name__ == "__main__":
    main()
```