There are several ways to configure the languages which are shown for you on an item.
If the selection of languages is wrong for you, perhaps it's wrong for everyone in your region?
If you think so, please help improve the data from which we guess which languages you may understand. After some weeks, such improvements to the standard reach not only you, but also all Wikidata users, all MediaWiki users and billions of users of other software!
Changing only for yourself
The extension records the last four languages you've selected using the Universal Language Selector (ULS), present at top of pages.
To add another language, make sure you've saved your current edits, then select the language with the ULS: it will become the default language used for the rest of the interface.
You can come back to your preferred language for the interface, either by using the ULS again, or by clicking one of the other languages listed in the top group, below your current language.