Looking at the initial implementation of the language selector (T253303) available for Section Translation at test wiki, the selector takes the whole screen width even when displaying at wider screens.

Based on the spec in T253303, the selector on wider screens is shown as a dialog over existing content with no header element (512px max width):

Screen size

• Narrow screen. The selector is shown as a full screen dialog. The list of all languages always uses one column.
• Wider screen. The selector is shown as a dialog over existing content. The list of all languages can use one or three columns depending on the number of languages (see above). The length of the selector will adapt to the available languages with a max height of 312px.
  
  |Wide screen|Narrow screen| |---|---| |
  
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