Demo import errors etc

Hey There

I have bought, downloaded Arcane theme a few days ago. To get familiar with it I have installed it locally and ran Demo installation, fresh WordPress installation followed by installing Arcane theme, all required plugins, then I used “One Click Install” option which gave me “Success! The data has been imported successfully.” information, so far all worked just fine, but here are my concerns and errors I noticed.

After importing demo data I can see quite a few things, will describe them as clear as I can:
1. Importing demo data change my WordPress language to English
Q: Is this intended or WordPress should stay with installed language?

2. Once demo data is imported I can not see much of it, site looks like this:
http://prntscr.com/d66ad0
I have checked with my friend who uses Blackfyre theme and after importing demo data website have quite much of the content, menu, pages, matches, team etc which I can not see on Arcane, this made me investigate from admin side what is exactly imported and I found following:

A. All the plugins are deactivated (they were active before importing demo) – this is not really a problem as all there need to be done is to activate them.
B. There is only “Create a tournament” page created, I think there are few pages missing.
C. Menu – there is 2 errors in this place http://prnt.sc/d66dmq
Warning: Creating default object from empty value in D:\xampp\htdocs\arcaneTEST\wp-admin\includes\nav-menu.php on line 485
Warning: implode(): Invalid arguments passed in D:\xampp\htdocs\arcaneTEST\wp-admin\includes\class-walker-nav-menu-checklist.php on line 113

I use XAMMP (ver xampp-win32-5.5.38-0-VC11-installer) on localhost for testing.
Checked on Firefox 49.0.2 (no addons/extensions), Google Chrome Version 54.0.2840.99 m (64-bit).
I have repeated process couple of times with very same results.

Can someone shed some light on the issue I’ve noticed?

Kind Regards
Greg