fixing navbar on one line

The more you enlarge a font, the less words are going to fit on one row. That is a fact. You need to consider:
1. using a smaller font-size
2. using a different, narrower, font
3. Changing your page names to shorter ones.
4. Any combination of the above.

Or:
5. reduce the padding between the menu items.

In your case, just playing around with the padding around the flag and the envelope will reduce it sufficiently. Something like this:

#menu-item-16, #menu-item-380 {
    padding-left: 0;
    padding-right: 0;
}

Thanks a lot for yours comments, I used them and delete a part of compromised css code, and all is ok.

thanks a lot for your help !