Accessibility: add aria-label to the button

Problem/Motivation

When I tested my project with Lighthouse, I got this:
Accessibility: Buttons do not have an accessible name
It failed this test because the element has no associated text.

Steps to reproduce

- Add a languages_dropdown to a project
- Open project in Chrome and run a Lighthouse test

Proposed resolution

Add aria-label="{{ 'Language selection'|t }}" to the element in languages_dropdown/templates/lang-drop-bootstrap-block.html.twig