I'm just now running into this too, as I'm trying to distribute on the Mac Store. The max length of a locale message is 112 characters for Safari Extensions!? That's a serious problem. For conversational messages it is not practical or advisable to break up the translations into small chunks. I often have entire paragraphs of help content that are translated to all my supported languages. Also I'm pretty sure my extension is working fine with these longer strings, so the validation error seems incorrect.

Okay, I see what's going on here. IF you use a "__MSG_<key>" based value in your root manifest.json for the description field, every _locales/*/messages.json file/entry referenced for this value must meet this 112 char limit. Presumably all the root manifest.json entries that use localized keys like this are validated as well. One oddity is that Apple is limiting the description to 112 chars but chrome's limit is 132. In my case I do meet the chrome requirements but not Safari's smaller limit.