One thing I don't know is how Google will cope with the mixture of Welsh and English on the same pages. It's possible that this 'confuses' it in some way, leading to worse search performance.
Voice adds lang="en" into the HTML header of the page, so search engines may well assume it's all in English.
You could experiment with creating separate pages for each language to see if this improves ranking of each in their respective language.
Or you could try wrapping the Welsh content in a div that indicates the correct language to see if this improves how search engines cope with it. You could use the HTML source button of the editor to do this.
You'd want to end up with something like this:
<p>Dyma'r cynnwys Cymraeg.</p>
<p>This is the English content.</p>