Fix character count with the better mb_strlen

[lanresmith]

trafficstars

Context

⛑️ Ticket(s): https://secure.helpscout.net/link/to/conversation

📓 Notion: https://notion.so/link/to/card

💬 Slack: https://slack.com/link/to/thread-or-message

Summary

i18n is crucial when developing WP plugins :). This fix provides a better character count. As simple as it is, a form submission could be prevented because of incorrect validation, which might lead to e.g. loss of sale or lead.