"Play Youtube Video" aria-label should be translatable

[audrasjb]

Describe the bug
When using image remplacement for Youtube Iframes, the "Play Youtube Video" aria-label should be translatable. This is currently an issue with Accessibility Guidelines and as far as I know there is now way to properly override this string yet.

To Reproduce
Steps to reproduce the behavior:

1. Activate lazyloading for video and activate thumbnails images for Youtube
2. Switch your installation to another locale, like fr_FR (Français)
3. Add a video on a post and confirm that the "Play Youtube Video" aria-label is not translated

Expected behavior
"Play Youtube Video" should be translated.

• You have to replace the current implementation to a regular gettext string so it can be translated
• Ideally, this string could be filterable via a hook (not mandatory)

Desktop (please complete the following information):

• OS: All
• Browser: All
• Version: current

Smartphone (please complete the following information):
Same

Additional context
The issue lies here: https://github.com/wp-media/wp-rocket/blob/develop/inc/Dependencies/RocketLazyload/Assets.php#L283

Cheers Rocketeers!
Jb