Made into separate ticket from T183178#3900548.
Language settings dialog is always displayed with wide width. Adding sidebar width on Vector skin, that results in 900px+ needed to display whole dialog. Trying to use smaller size (medium) gives somewhat usable UI, but we may need new design for smaller widths layout. ULS list of all languages (clicking on ... button) should open with same width as language selector was (for example, if language selector was having medium width, ULS should open medium as well).
Here is how it looks if we change wide class to medium class. The min-width rule makes it look like this until 1250px+, when percentage width rule kicks in.