SEO can be magic ("Some may say..."), which I don't like, but - certain things are certainly more objective. Having a prefix vs. no prefix? Only matters possibly in terms of diluting keywords located in the URL. If it's 2 chars, I'd not worry about it. The other concern is more canonical (i.e. authoritative page for content, in case there is dupe content within the page). Personally I'd suggest consistency between all languages, but that's me.
-- tangent --
On another note (and possible tangent): I literally just dealt with overriding 404 pages to facilitate handling a custom (complex) redirect, in case a page was imported (from old version of the site) and moved to another location and that was a huge PITA. In this case, that 404 may still have an English language equivalent that you'd like to failover to -- how do you do that? Well, that's a good question and I'm very glad you asked, and Welcome to SilverStripe! To allow 404's to still 301 redirect to a new location (if that were something you needed) this might be how you'd do it (I'm sorry I know how to do this):
- Define a CustomErrorPage_Controller which extends Page_Controller
- Define a "CustomErrorPage_Controller" in your YML config as an override
- In the handleRequest() method, you'll want to just check to see if the current $this->ErrorCode is == 404 and then throw in custom logic there, but wait, there's more to it...
- Create another class called CustomSS_HTTPResponse which extends SS_HTTPResponse. Why? Because...
- If in the ->handleRequest() above you need to redirect and you return a response to redirect, the code get's clobbered and overridden to 404 again, which we cannot have.
- In your new CustomSS_HTTPResponse class, just ensure that once the ->setCode() method is called, only allow changing the code if currently still the default of 200
-- end tangent --
Phew -- just FYI incase you do want to use the 404 route to failover to a 301 for a better page alternative. From a cultural standpoint, I'd be willing to wager that a reasonable amount of the "nl" speakers would at least be likely to understand "en". Only caveat there is that now their experience has just been converted to entirely being English which they obviously may not prefer.
- Patrick