How to optionally capture a path segment in Haskell servant?

I need to introduce a internalization in my servant app in a backward compatible manner. I want to have the language code as an optional path segment. If the very fist path segment is from a known list of language codes, that language is used, else it defaults to Nothing (which the handler/server would interpret as some default language).

/some/path => myHandler Nothing
/en/some/path => myHanddler (Just "en")
/fr/some/path => myHandler (Just "fr")

To avoid having to write the following boilerplate…

• Each endpoint will have two endpoints: one with the language code segment, and one without
• Therefore, the server/handler will also need to have two variants — one with language code and one without

…I wanted to come up with a servant combinator that “peeks” at head $ pathInfo req and it if looks like a language-code, it captures it, else it passes along Nothing to the sub-server.

I came up with the following, but can’t get it to type-check because I’m not sure how to use addCapture along with passToServer

data MaybeLanguageCode = MaybeLanguageCode

knownLanguageCodes :: [Text]
knownLanguageCodes = ["en", "fr", "es"] -- etc

instance (HasServer api context) => HasServer (MaybeLanguageCode :> api) context where
  type ServerT (MaybeLanguageCode :> api) m = (Maybe Text) -> ServerT api m
  route _ context subserver = 
    route (Proxy :: Proxy api) context (passToServer subserver extractLanguageCode)
    where
      extractLanguageCode :: Request -> Maybe Text
      extractLanguageCode req = 
        case pathInfo req of
          [] -> Nothing
          (x:_) ->  if x `DL.elem` knownLanguageCodes
                    then addCapture _todo -- nothing that I do here will type-check, it seems
                    else Nothing