Blocs is vastly more advanced that iWeb was in terms of flexibility, but it can also be used in a simple manner if your needs are modest. More advanced users will be creating custom classes for very fine control over every aspect, however many will just rely on the settings in the side panel.
I am little confused by your talk of not putting this on the internet. Your work can always be viewed inside Blocs or previewed on a browser locally without ever uploading to the internet but I think it is reasonable to say this is primarily a web design app, where the typical destination for a project would be on the internet. If you really intend to avoid the web entirely there are probably different tools available and I would have said the same about iWeb.
Inside the app itself the interface is all in English. You can set English as the language in project settings, but this is only of relevance to browsers and Google on the internet. Blocs is developed here in England but you can write any way you like inside Blocs. There is no option to set British English or US English as such. If you are feeling Brave it has options for Cornish or variations of Gaelic, but none of this will have any affect on what you see.
There is a free demo version available if you want to give it a spin. https://blocsapp.com/