Fix date formatting issue in pop-up window

[This may not be the current release; go to the releases page to ensure that you're downloading the current release.]

Fix the problem which sometimes caused Send Later to insert a date into the text field of the scheduling pop-up that it was then unable to parse, or that it parsed incorrectly (e.g., swapping the month and the day). Although this problem should be mostly gone, it's possible that it may rarely continue to show up, so a detailed user guide section has been added to explain what to do if that happens.

Don't save bad custom date formats. In other words, if the user goes to customize their date formats in the Send Later preferences, the preferences tab will refuse to save any formats that don't parse properly.

Add a few translation updates.