Unable to find out how to edit Titles from horizontal menu [prestashop 1.6]

What I am trying to show below is a workaround for v2.2.1 of ps_mainmenu module installed in PS 1.7.5.0. I don't think there is a difference with the version you are installing in your PS 1.6.

In the module configuration page, there is a part enabling you to create a link (second part). It contains 2 main inputs : Label and Link.

In label, put Výrobcovia. In link, put the link to manufactures page, e.g, if the current manufacturers link is localhost/abc, add it in the input.

Then click on add.

You will see your new link in the first part of config page (like in your first imgur). Add it to your menu in the left part.

Delete the Všetci výrobcovia.

There is for sure a solution that consists by editing the module or by adding a translation. I will be back here if I find it.

Edit You can see from the module code source that there are many places where the string 'All brands' is translated. The module uses this string instead of 'brands'. Since it is discouraged to modify built-in module files, I would go and change in the translations by putting Výrobcovia as translation of 'All brands'.

I hope it helped.

I think that you mean a module Top horizontal menu (but in other cases the way should be the same). The easiest way to achieve your goal is to manage your translations. In your admin panel go to Localization -> Translations and in a section Modify translations in the first select choose Installed modules translations, in the second choose a theme for which you want to change the field or core if you want to change it for all themes, in the third choose a language you want to change and press Modify button. In the result page, find your module and expand a translation section. Find your variable inside and change it as you wish