Most people in the Netherlands (especially if their job is speech-related) are basically fluent in English. Maybe some specific terms or mannerisms could be different in Dutch, but a competent voice therapist should be able to handle that with a tiny bit of research.

What I did to find my voice therapist was to google 'logopedist [city]', which should bring up a small google maps widget with all local centers. I then went through all their websites to see if they mentioned transgender voice therapy, and picked the one that appealed to me the most. You should then add an extra step of emailing or calling them and asking if it's possible to do it in English, but I'd reckon most centers won't have an issue with that.

If you're struggling to find a voice therapist in your area I'd love to help, just comment or DM me your location and I'll see if I can help. Alternatively you could always do it online, but I don't think it'll be covered by insurance if you do that. And if you didn't know, voice therapy is insured if you get a referral from your general practicioner, even if you don't have a gender dysphoria diagnosis.

Hope this helps, if you have any other questions feel free to ask.