Tugboat demo: add French as a switchable language to review the UI in French

The Tugboat preview ships the demo UI in English only, so a reviewer cannot easily check how the pdv interface reads in French. This adds French as a second, switchable language to the demo seed.

What it does

• Installs the language and locale modules and adds French. English stays the site default, so the demo is unchanged for English reviewers.
• Imports the French translations bundled in each enabled pdv module's translations/ dir, so the pdv vault dashboard, consent ceremony, owner passphrase, operator surfaces, and Webform vault elements render in French.
• Adds a language switcher block (Olivero secondary menu) so a reviewer can flip the interface to French on demand, then back to English.

Why the .po files are imported directly

pdv has no localize.drupal.org translation server, and the import core would otherwise run on module install is a batch. That batch does not execute under drush php:script (the context the seed runs in), so the bundled .po files are imported directly with Gettext::fileToDatabase, the same core importer the batch would call.

Verification

Ran the full seed against a standard install: English remains the default, French is available, the pdv strings resolve in French (for example "Your vault is locked." becomes "Votre coffre-fort est verrouille."), and the switcher block renders working English and French links.