I believe the most straightforward approach is to remove language versions directly in Sitecore databases, followed by rebuilding all caches and indexes.

1. Execute the following queries on your Master and Web databases:

DELETE
FROM [VersionedFields]
WHERE [Language] = 'fr-FR'

DELETE
FROM [UnversionedFields]
WHERE [Language] = 'fr-FR'

2. Clear item caches on all instances using the page /sitecore/admin/cache.aspx

3. Rebuild the link database on all Sitecore instances. You can do that from the control panel.

4. Rebuild search indexes on all Sitecore instances. You can do that from the control panel.

In addition to other answers, you can do this manually by removing language versions directly from Sitecore databases, followed by rebuilding all caches and indexes.

1. Execute the following queries on your Master and Web databases:

DELETE
FROM [VersionedFields]
WHERE [Language] = 'fr-FR'

DELETE
FROM [UnversionedFields]
WHERE [Language] = 'fr-FR'

2. Clear item caches on all instances using the page /sitecore/admin/cache.aspx

3. Rebuild the link database on all Sitecore instances. You can do that from the control panel.

4. Rebuild search indexes on all Sitecore instances. You can do that from the control panel.

Be sure to test this thoroughly in a test setup, since modifying Sitecore databases directly may sometimes break your site.