Language Selection missing on the 'Under maintenance' front page

[samz]

In the 'Under maintenance' front page, the language selection is missing. So I have to add links manualy like this:

<p>We are currently updating our shop and will be back really soon. Thanks for your patience.</p>
<p><a href="../fr/" class="btn btn-default">FR</a></p>
<p><a href="../de/" class="btn btn-default">DE</a></p>
<p><a href="../it/" class="btn btn-default">IT</a></p>
<p><a href="../es/" class="btn btn-default">ES</a></p>

What is the PHP loop code adding links (BOOTSTRAP button style) to all available languages on this page?

As you can see on my image, the 4 links doesn't look like BOOTSTRAP button style, defined by: class="btn btn-default"
The BOOTSTRAP files are note linked to the shop when I look the source of the page localhost/fr/

How to fix this?

Thank you in advance and regards,

PRESTASHOP 1.7.6.7 on localhost

Edited September 27, 2020 by samz (see edit history)