Content for textgroup views is not allowed for translation because of its text format
I had the same problem, then found a solution. I'll show you the details with screenshots.


First enable Internationalization Views (i18nviews).
Create your custom view. For example, I'll simply list the nodes related to Article content type.
Add a header:


Click on "add" next to Header section

Add "Global: Text area"

Type your header text, and select your input format. Please note that I selected "Filtered HTML" here!

Click Apply 

After saving the view, you'll see notice, telling that this header text can not be translated, so in my case:
"The string views:list_articles:default:header:views:area:content for textgroup views is not allowed for translation because of its text format."

Go to admin/config/regional/i18n/strings (Home » Administration » Configuration » Regional and language » Multilingual settings » Strings)
Check the appropriate text format you used above, when adding a header text, it must be the same - so in my case, it's "Filtered HTML" -, and then click Save configuration.

Now go back to your custom view, and on the right dropdown menu, click "translate view":

Now you'll see this:

Click on one of the "translate" links. After that, you'll see the following screen, and a message that the appropriate string for the header text translation has been created - which means from now on, you can translate it!

Scroll down to the appropriate header text, and translate it:

After translating, click on "Save translation".
On the following screen, you can see that some translations have been saved.
Now when switching to the other language, you'll see that the header text is translated! You are READY!