I am running impress and use English. My operating system is Windows 10 and that is also English. Have just downloaded 5.4.3.2
You didn’t give an example of what you’re trying to achieve.
I assume you have a TOC automatically extracted from your text, i.e. made of paragraphs which have received a Heading x paragraph style (x from 1 to 10).
TOC entries are controlled by paragraph styles Contents x. There is a one-to-one mapping between heading and contents levels.
Leaders are associated with tabs. They are composed with a “fill character” as it is called in the tabulation dialog. Built-in fill chars are dot, hyphen and underscore but you can use any you like.
If you’re satisfied with the built-in layout offered by Contents x but for leaders, open the “offending” contents style for modification (press F11, right-click on style name and choose Modify…). Go to the Tabs tab. Select the tab position for which you want a leader and choose the fill character. Press OK.
If you want to change the layout, the simplest way is to customise it through Insert → Indexes & Tables → Indexes & Tables, tab Entries. Tabs are identified by T descriptors in entry template. After that, choose the fill character in the corresponding Contents x paragraph style as above.
If this answer helped you, please accept it by clicking the check mark 
to the left and, karma permitting, upvote it. If this resolves your problem, close the question, that will help other people with the same question.