SEO questions

Hello,
After checking with Google webmaster tools, I found that my site, https://www.petrutech.com has many problems, and it looks that I must make many changes, actually rewriting many pages. This is a 6 language site, and I began with the duplicate description. It looks that is more than one month job... :(
I have several questions, and I'll be very thankful for assistance.

1. One of the duplicate problem is the following:
   ¡Zen Cart! : Nuevos productos - Transmisores de habitacion VHF Transmisores por Telefono Receptores
   /products_new.html?amp%3Bdisp_order=6&amp%3Blanguage=sp&disp_order=6&page=2&language=es
   /products_new.html?disp_order=6&language=es
   I am trying to find this page for 3 days, without success :(. I wish to fix the duplicate text (meta description) and title, also the %3B. I cannot find it anywhere.
2. Do I need to use the hreflang? If I have to do it, where do I have to insert it? in the general description? I got the following info:

The hreflang tag can be placed in the on-page markup, the HTTP header, or the sitemap. Only use one of those locations. If you choose the sitemap, this tool can help you.
I don't know where to find the on-page markup or the HTTP header.

3. Links... All my external links come from clips that I publish on Youtube. I assume that there are not enough - is there any way exchanging links with quality sites?
   Thank you in advance for any help / assistance!

Your site's URL rewriter is "mapping" /products_new.html to /index.php?main_page=products_new, i.e. that page's contents are generated by the Zen Cart products_new processing.

One observation: You've somehow got both a **sp **and an **es **language identifier (I'm guessing that they're both referencing your Spanish translated pages). I believe, but do not know, that the multiple language references might be confusing your SEO issue.

@mprough has an article that describes how to add the hreflang codes to those canonical links.

@lat9,
Thank you very much,
Unfortunately, my knowledge in this field is very limited. However, I am trying to do my best, and also try to fill this gap in my ignorance.
This site was initially V139 as far as I remember. I've updated it twice (I couldn't do it by myself, I've used the services of a third party).
Also, the last guy who fixed it, made it mobile responsive.

As far as I can see, I have many "meta description duplicates", and I found many other mistakes, all done by me. Meantime, fixing them goes very well, but these two links frustrate me. While I can edit one of them (I suppose that it is /products_new.html?disp_order=6&language=es, I must find where is the second one, and I believe that maybe it has to be deleted.

I've d/led the entire site on my PC, but I don't know where or what should I look for.
If you may have any idea how to fix it, I'll be very thankful.
Thank you!

You might try looking in your admin's Localization->Languages just to make sure that there's not a "rogue" ***sp ***language defined there.

Thank you for the suggestion. Nothing found so far, it is hidden somewhere for sure, but I cannot detect it.