As discussed, we need to replicate this issue on a fresh WordPress installation. Then, I'll provide you with the needed code or escalate it to our compatibility team. For this, I created a test website with a clean WordPress install. You can access it through this link:
hidden link
With this link, you'll be directly logged in.
Could you please install the Spectra plugin and see if the issue is replicable on a clean WordPress installation?
Please configure WPML as set on your site (just the basic configurations needed to replicate the issue are enough) and ensure you install the latest version of the plugin.
Please let us know when everything is finished, and you can replicate the problem.
Important! Do not import your site to the test site. We must replicate the problem on a fresh, clean WordPress installation.
I recreated the problem in the sandbox - please have a look. I created the page in german, then translated it to english. I have not found the possibility to translate the term "Überschrift/table of content" in the strings. As you can see, in the table of contents on the english page it is not translated.