Is there a website in English where you can purchase theatre tickets for the DeLaMar Theatre, for example?
Most sites have an English option, but I'm thinking that as a Dutch Language theater, they're thinking, if you don't understand Dutch, what's the point? You can always use the Chrome translate function.
For theater tickets anywhere in the world, it's always best to purchase directly on the link from the theater's official website, to avoid resellers and their extra fees. (There can still be fees on the official ticket site, but these will still be lower than a reseller's fees).
Dutch theater ticketing websites may only be in Dutch. In that case, you can use Google Translate. It's built in to the Chrome browser, or you can get it from https://translate.google.com/. It's supposed to auto-translate, but I find it works best if I choose the languages (in this case, Dutch to English). It's not totally accurate, but should get the job done for this purpose.
Out of curiosity, I just checked for the De La Mar theater. If you go to their website https://delamar.nl/ in Chrome and let it translate the page, then choose Your Visit, then Frequently Asked Questions, one category is "Buying Cards" (see what I mean about the translations not being 100%; it really should say "Buying Tickets"). If you click that, it tells you how to buy tickets, in detail.
Unless you speak Dutch, I assume you're interested in Fiddler On The Roof or Sweeney Todd. Go for it - I saw great productions of Sweeney Todd in Barcelona (in Catalan) and Berlin (in German). Since I already knew the show, I enjoyed these productions thoroughly.