OK...
- Is as I thought, it's not an error as such, because you have debugging on (can be checked at Administration > Site administration > Development > Debugging ) it's just telling you the language file has been found. It's OK to continue installing.
- I really don't have a clue about I'm afraid; hopefully someone more knowledgeable will be along soon.
I "think" you might be able to get around it by heading to Site Administration > Server > Update notifications and switching it off until you install the plugin manually.
Also, could be a good idea to look into a browser plugin for screenshots so you can insert them in your posts (to be honest, normally I don't open attached files when I am browsing through these forums unless the issue really interests me or, as in this case, I'm already involved). The one I tend to use is Awesome Screenshot due to the mark up abilities and it's available in Chrome and
Firefox.
Edit: In fact re 2,what happens if you hit "Continue"? When you have debugging on things that normally you don't see will stop the script so perhaps it's just saying that when it checked to see if there was an update the reply was empty (understandable as it's a new plugin so wouldn't have data top check against).