Subtitle translation

Hi everyone,
I use the Forever theme and I wanted to add text under my header image, so in my CSS I added this :

}

#masthead .custom-header:after {
	content: "SUBTITLE";
	color: #0B173B;
	font-size: 23px;
}

#masthead .custom-header:hover {
	text-decoration: none;

I wanted to know how to translate the content (“SUBTITLE” in this example) ? I tried adding [:en] and everything but it just doesn’t work.
If someone can help me, that would be nice!
Thanks a lot!

https://wordpress.org/plugins/qtranslate/