editing a split header

Site URL: https://www.gaisi.org

I just started my new job, where I am in charge of a site that somebody else made. Therefore, I do not know what codes are used to customize the site. The site is available in two languages, where the header is split in to two separate parts, with each language on separate sides. 

I now need to ad a new page to my header, one to each side. So a total of two new pages will appear in the header. The problem is, I am not able to put the page in to each side of the header.

I suspect there has been added a code, but I can not find this code. Can anyone help me?

Thank you in advance. 

The menu itself is pretty simple. It's a regular menu with one of the elements having a lot left margin. In Pages > MAIN NAVIGATION I think you'll find the items listed in the order displayed below. You need to put your new pages in main navigation and order them how you want. Then adjust the ruleset mentioned below.

The ruleset for that element is in Design > Custom CSS and is as follows.

.header-nav-item:nth-child(5) {
    margin-left: 25vw !important
  }

Thank you for your help! I edited the code, and it is now correct. But, the menu for phone-viewing is not. How do I fix this? As you can see in the photos, the Oddest button is on the left side in computer viewing mode, but note when viewing on mobile phone.  

21 hours ago, vilgut said:

but note when viewing on mobile phone.  

This is the ruleset you need to update.

.header-menu-nav-item:nth-child(5) {
  border-bottom: 1px solid;
  padding-bottom: 1em
}

.header-menu-nav-item:nth-child(6) {
  padding-top: 1em
}