Hi extremecarver… Here is what I experienced on my test site while trying to edit an existing photo in my Media Library:
The Title, Caption, and Description fields had the <!–:en–> language tags, and the Alt field had the [:en] language tags (no closing [:] tags, since these did not exist prior to 3.1).
First, I tried to simply open the photo in the full-page Media Editor and hit the Update button without making any changes. This left all fields exactly as they had been–no changes at all to the language tags–and page layout still broken due to the Alt closing quotes issue.
Next, I loaded the photo back in the full-page Media Editor and “pretended” to edit the Alt field for each language. What I actually did was place my cursor at the end of the Alt field text, hit backspace once, and then retype whatever character I had just eliminated using the backspace key. I did this for each language, hit Update, and my Alt field now had the [:] closing tag at the end of its string, which fixed my layout problem. You could accomplish the same result by opening the “popup” photo editor and simply typing [:] at the end of your Alt field string.
Since I made no changes to Title, Caption, and Description, they stayed just as they were with the <!–:en–> language tags, but they didn’t appear to cause any issues with my site so it would seem it’s OK to leave those fields alone.
If you do make changes to these fields using the full-page Media Editor (not the popup), then yes, the <!–:en–> language tags for the edited field will be replaced by the [:en] language tags and a [:] will be placed at the end to close. This is only true for fields you edit. So, for instance, if you change your Title, its tags will be converted, but Caption and Description tags will remain the same unless and until you edit them in the full-page editor.
Note: I only tested scenarios where ALL photo fields had values. There are times where WordPress substitutes Title and/or Description for the Alt attribute if the actual Alt field has been left empty. I wouldn’t anticipate any issues in such situations, but I did not have time to specifically test this scenario.