This question has been answered by sottwell. See the first response.
If you are going to manage your languages with resource tree structure, you might be interested in this https://forums.modx.com/index.php/topic,29597.msg179898.html#msg179898
Basically, at its simplest, you need to have two "parent" resources in the root level, one for each language, then you use UltimateParent as the startID for your Wayfinder snippet. This will keep the menu structure limited to the branch of the resource tree for the current language.Sounds exactly what I need but forgive my ignorance how do I create two parent resources? I currently have all pages in what appears to be a folder called web (the hover state tells me it is the 'default front-end context for your web site'. as you will realise I am a complete modx novice and the terminology is proving harder to master than the CMS itself.
All the rest of the stuff is just to establish which language has been selected under different circumstances.
[[Wayfinder? &startId=`[[UltimateParent]]`]]