How to define a captions language by default ?

Ok I found how to make this.

In file mediaelement-js-wp.php :

Replace :

// captions
		'captions' => '',
		'captionslang' => 'en'

by :

// captions
		'captions' => '',
		'captionsdefault' => '',
		'captionslang' => 'en'

Add :

if ($captionsdefault) {
		$captionsdefault_source = 'startLanguage: "'.$captionsdefault.'"';
	}

Replace :

{$loop_option}
		{$controls_option}
		{$audio_size}

By :

{$loop_option}
		{$controls_option}
		{$audio_size}
		,{$captionsdefault_source}

Now, you can use the new tag captionsdefault like this :
[video src="XXX" captions="XXX" captionslang="fr" captionsdefault="fr"]