Button text translation

I’m using this plugin in combination with WPML. I was unable to translate the text on the front-end when the user was not logged-in. Really strange. In the end I found the issue: I had to enable language filtering for AJAX on the WPML Languages settings page. More info: https://wpml.org/errata/unable-to-get-site-language-on-the-front-end-ajax-calls-when-user-is-not-logged-in/.

But the button text still doesn’t translate. I hardcoded the text to Contact, which can be used (in my case) for three languages. The text should be translatable (it has a __() function) but it doesn’t work.