The plugin is vulnerable to an authenticated SQL injection. By adding a new language (via the settings page) containing specific special characters, the backticks in the SQL query can be surpassed and a time-based blind payload can be injected.
Proof of Concept
To exploit the vulnerability, someone must send a specifically crafted request adding a new language containing specific special characters, and then open another page and measure the response time to retrieve data. This can be automated via sqlmap.