How to display the widget flags horizontally?

The appearance of your site is controlled by CSS rules placed in the file style.css in your theme directory. The appearance of the language switcher is controlled by the classes “lang-item” for all languages and “current-lang-item” for the current language.
So you can add the following lines in style.css

.lang-item {
  display: inline;

The topic was also discussed on the support forum: Flags vs. Text Links in widget