Since 1f15d1d5828dfbc70d036a52da57460ea0e89175 the link to the watchlist on the watch success notification message is not localized anymore:
Example of the link on the watch success message on dewiki:
Before 1f15d1d5828dfbc70d036a52da57460ea0e89175: https://de.wikipedia.org/wiki/Spezial:Beobachtungsliste
Since 1f15d1d5828dfbc70d036a52da57460ea0e89175: https://de.wikipedia.org/wiki/Special:Watchlist
Server-side parsed messages (T27349: ResourceLoader: Support loading of messages in parsed formats (e.g. parsed, incontentlanguage, ..)) would be useful here.
The link comes from the appropriate i18n message, which uses the canonical name of the special page so it can function correctly even when you're using German on a non-German wiki. When parsed server-side MediaWiki turns it into the local name for the link, but the JavaScript message handling can't easily do that.
Either link still works, so at worst this is cosmetic and slightly less efficient due to an HTTP redirect from the canonical title to the local title. If it bothers them that much, dewiki could always override the i18n message locally to use the local special page name instead of the canonical one.
Personally, I'd recommend this be closed as "Declined", but I'll let others make that decision. But I will -2 any attempt to undo the relevant part of rMW1f15d1d5828d: API: More i18n cleanup.
rMW1f15d1d5828d: API: More i18n cleanup is the right direction. But some functionality are lost and this is a step back. It is not wise to implement the functionality on client-side. But with server-side parsed messages the lost functionality can restored.