un activate

Hi!
Interesting plugin!
By the way, I tried to activate but failed.
The error message was,
Parse error: syntax error, unexpected ‘[‘ in /home/hosting_users/…/www/…/wp-content/plugins/set-html-lang-attribute-per-post/set-html-lang-attribute-per-post.php on line 105

Any solutions?
Thanks in advance.

https://wordpress.org/plugins/set-html-lang-attribute-per-post/