Hi, thank your for your suggestion. I have not understand as font weights don't work since in x-theme they are loaded. I can see all Assistant weights in theme options.
So, if I call a specific weight in my css it should be work. In the past if I used a font weight already present in x, it was working.
It's not clear how it could not work.
And then, I have checked the link you have pasted for load Google font in the function.php and those of WpBeginner says the best way to load GG font is to use the direct link in head tag.
Performance Optimized Method of Adding Google Web Fonts
The best way of adding Google fonts is by using the Standard method which utilizes the link method instead of the import method. Simply take your font URL that you got from step 1. If you are adding multiple fonts, then you can combine the two fonts with a | character. Then place the code in your theme’s head section.
You will most likely have to edit your header.php file, and paste the following code above your main stylesheet. The example would look like this:
1 <link rel="stylesheet" type="text/css" href="http://fonts.googleapis.com/css?family=Lora|Oswald" media="screen">
2 <link rel="stylesheet" type="text/css" href="YOUR THEME STYLESHEET" media="screen">
Basically the goal is to put the font request as early as possible. According to the Google Web Fonts blog, if there is a script tag before the @font-face declaration, then Internet Explorer won’t render anything on the page until the font file is done downloading.