If a source message has been changed and needs updating, Special:Translate will show one or several warnings for the message. Only one warning is shown by default, while any additional warnings are collapsed. One of the warnings (when the source message is changed) is always the one saying "This translation may need to be updated. Show differences". However, if the message has gotten more new parameters, the warning for missing parameters is the first warning showed, while the one with "Show differences" is collapsed.

I think this should be changed so that the warning with "Show differences" is always the primary warning if it is present. Usually the reason parameters are missing is because the parameters weren't present in the previous version of the message. So the most important way to find out why a parameter is missing is to see the diff.

Some screenshots:

• 
  Default view when you start editing a message that needs updating
• 
  When you click the small "1 more" toggle
• 
  When you click "show differences" to see what you actually need to see.

Additionally, I would prefer to see the diff immediately instead of having to click "Show differences", but that could probably be considered a separate issue.