How to change default interface messages and text in MediaWiki
The interface text consists of predefined text strings that come prepackaged with MediaWiki and that are the same on all the pages on which the particular text strings are shown and for all users. This is pretty much all the text that's not the content of the articles. This includes the text for the different buttons, menus, options, special pages, error messages, etc.
If you want to, you can change these default text strings with something else of your choice. You can do this from the frontend of the site. Each text string has a corresponding page from which you can change that text string. You can access that page directly with its URL address provided that you know the message key that corresponds to the text. Another way to access such a page is to use the special page that lists all the default interface messages. From there you can access the page of each text string and edit it.
Only users who belong to a user group with the editinterface right can change interface messages. By default, only admins/sysops have that permission.
Check and View Message Keys
The list on the special page with the default text strings is quite extensive and if you don't know the corresponding message key of the text you may spend some time looking for it. There's an easy way to check and view the message key behind each default text string on a page. To do this after you open the particular page just add &uselang=qqx to the URL of that page and open the new URL. All the default interface text strings will be replaced with their corresponding message keys; they will be put in brackets. For instance, the message key for the Login link is pt-login.
Edit Interface Text
Each interface text string has a page from which you can edit it and change it. One way to access such a page is to directly open its URL address. To do this you have to know the message key corresponding to the text string. You have to add MediaWiki:name-of-message to the main URL of your site (name-of-message has to be replaced with the actual message key). For example, the message key of the Preferences link for accessing user account options is mypreferences. If the URL of your MediaWiki site is yourdomain.com, then to open the page of that interface message you have to open the URL yourdomain.com/index.php/MediaWiki:mypreferences. One the page that opens as the content of the page you'll see the text string as it appears on the frontend; in our example this will be the text Preferences. You can change the message by editing the page. Just edit it as any regular page, delete the old text and type the new one as you want it to be displayed to all users of the site.
There's also a special page that lists all interface messages. From there you can access the page of each message and change it. To access this list, go to Tools menu on the left>Special pages>Data and tools section>System messages button. This will open a table with the system/interface messages. The list is quite long and is divided into many pages. There are filtering options at the top. If you know the corresponding message key, you can use these options to search for it. The table with the messages is divided into two columns. In the left column you'll see the message key and in the right column the text that's shown for it on the frontend. To edit a message click on the name of the message key in the left column. This will open its page. Edit the page and change the text with whatever you want.