Good day Sietse, thanks for your question. I am glad to be able to provide assistance to you today. It looks like this feature is not working as intended as for you to be able to change the language when you right click on a selected text is that you will need to click on the translated button (with an a symbol) on the right side of the address bar and from there you will need to select a language first and then it will translate first the whole page instead of the text that you selected and then once you click on show original then you can right click on a selected text then that is the only time that you can use the language that you have previously selected and my apologies as well as for this it will be best to report this as well to the team that is working on Edge so that they can correct this feature as the languages option should be available when you right click on the selected text and for this hopefully a fix will be applied soon on the next update and you can provide a feedback from the browser itself by clicking on the 3 dots > Help and feedback > Send feedback.
Regards,
Paul R.