7z is very misleading when working with sitecore package.zip
that file is not entirely zip compatible files below won't be unziped
\package\metadata\sc_comment.txt
\package\metadata\sc_packageid.txt
\package\metadata\sc_poststep.txt
\package\metadata\sc_publisher.txt
I assume one of those files contains essential data how to read language and version number in sitecore items. You can find the culprit in \package\installer\project file, it will look like this:
<x-item>/master/sitecore/content/Home/Products/Europe/Independent Breaks and Mini TRIPS/World War 1 Battlefields/Summer 2015/World War 1 Battlefields/Sightseeing highlights/1 Vimy Ridge/{694AB1C9-F690-445F-9E4A-2F1BD0CC1E09}/invariant/0</x-item>
Notice end of the string. Invariant should be replaced with language and 0 with version number but because that information is missing you get the error: Could not parse the language '1'
This is reason one why you should not tamper with package.zip
But if you have to I've found a workaround how to add your files to existing package.zip. Unfortunately with some limitations.
I had to create a package on top of sitecore package but my package contained only dll's and config files (package without serialised sitecore items). Sitecore package was too big to manually recreating it so I used sitecore rocks package manager to create my package and then merged it with sitecore package.
To achieve that I had to merge
\package\files
\package\installer\project
and
\package\properties\files
You can safely add regular files like dll's or config files to original sitecore package.
Open package.zip in 7z(don't unpack) add your merged files and folders to that existing archive(drag&drop). Leave the package.zip and 7z will ask you to overwrite. That doesn't touch Sitecore files hence it won't corrupt the package.
I am not sure if that method will work with serialised items but I have a hypothesis that if you replace /invariant/0 to something like /en/1 it will be able to install that one language version and you woudn't need those nonzipable Sitecore files.
You can also skip using Sitecore rocks package manager but I think it helps to generate files and understand what package.zip consist of.