WPML – Change media uploader language

I am french too, but I must answer in English.
You need to use wp_localize_script(), that allow to handle localization in js scripts.

add_action('wp_enqueue_scripts', 'load_script');
function load_script(){
    wp_register_script( 'popup', plugins_url('yourscript/js/yourjs.js'), array( 'jquery' ), false, true );
    wp_localize_script(
        'popup',
        'popUp',
        array(
            'popupTitle'=> __('', 'your-text-domain)
        )
    );
    wp_enqueue_script('popup');
}

In the js file, you will able to retrieve value:

function save_image($id){
    var mediaUploader;
    event.preventDefault();

    // If the uploader object has already been created, reopen the dialog
    if (mediaUploader) {
        mediaUploader.open();
        return;
    }

    // Extend the wp.media object
    var popupTitle = popUp.popupTitle;

    mediaUploader = wp.media.frames.file_frame = wp.media({
        title: popupTitle,
        button: {
            text: popupTitle
        },
        multiple: false,
        type: 'image'
    });

    // When a file is selected, grab the URL and set it as the text field's value
    mediaUploader.on('select', function() {
        attachment = mediaUploader.state().get('selection').first().toJSON();
    });

    // Open the uploader dialog
    mediaUploader.open();
}

https://codex.wordpress.org/Function_Reference/wp_localize_script

You can install the plugin WordPress Media Javascript Guide. It explains with screenshots and examples the must use method of the wp.media object.

Just a snippet to change the button text and the title:

wp_register_script( 'media-bbss-uploader-js', BBSSLIDERS_DIR_URL .'/js/bbss-multiple-media-upload.js', array('jquery') );
wp_localize_script(
    'media-bbss-uploader-js',
    'screenHelp',
    array(
        'title' => __('Select or upload new slides', $this->plugin_text_domain),
        'buttomText' => __('Add slides', $this->plugin_text_domain),
    )
);
wp_enqueue_script( 'media-bbss-uploader-js' );