Your challenge is for the English language to be a co-equal language in Germany by the year 2000. That means English is equally widely spoken in society, business and politics as German. I'm assuming we'll need a POD before 1900 to do this.
Bonus points if you can get English ascendant.
If nothing unforeseen happens (but this is of course never the case) you might see a situation like this in 2100 or maybe 2150 anyway.
If you change history enough to allow for an important role of English in Germany, you`ll most likely alter at least one of the languages involved, and, more importantly, you`ll alter at least the way Germany looks politically, and given Germany`s role in 20th century history, perhaps much of the rest of the world, too.
I don`t think the Hanover monarch idea is going to work because that`s rather late and German nationalism and national identity have developed roughly the way they had IOTL.
You need an earlier PoD - and a reason why English influences in Germany won`t just lead to a new blended language (like Anglo-Saxon and Norman French blended into Middle English).
One way to have two languages in Germany is to have some part of Germany speak English, while the rest speaks German. In the world of today, this could prompt general bilinguality, like Canada`s English-French bilinguality.
I´ll spare us the terrible option of the Nazis conquering the UK and winning WW2, in which the British Isles became an English-speaking part of "Germany"...
... and instead go for a more long-lived Hansa which seeks the support of the English Kings, instead of warring with them and obtaining a Pyrrhic victory in 1474.
If this Hanseatic-English alliance, which begins against Dutch and Danish competitors, is to last into the new age where America is discovered, it´ll have to be directed against other competitors like Spain, too. The Dutch could turn into allies at some point - perhaps with the reformation... and then become enemies again, perhaps because of Reformist-Lutheran/Anglican divisions or for some other reason.
In such a scenario, you`d have a lot of city states in Northern Germany (and elsewhere) where, once linguistic standardisation kicks in, it´s not High German which becomes the norm but Low German (which is pretty similar to Dutch BTW). Let these city states develop their own national identity - and let it be differentiated enough to maintain the differences that prevented them IOTL from becoming a major power, so that when national states become the powerful norm everywhere in the 17th and 18th centuries, most Hanseatic towns look for England / Britain for protection (instead of the atrophied HRE). Depending on how tough pressure from the surrounding states (Sweden, Denmark, Prussia) is, Britain might be able to absorb them temporarily into the Empire, maybe for a century or so, in which English becomes predominant over Low German.
How to bring them back into Germany?
Either this way:
At some point in the 19th century, a military unification of Germany as it occurred IOTL might absorb all or most of these British Hanseatic possessions (the equivalent of the 1870/1 war would have to be conducted against Britain, then, instead of France). During the age of nationalism, High German would be enforced upon the population, but after WW2 (if it still happens in this ATL), English (and Low German) would be granted the status of additional official language in, say, Hamburg, Bremen, Lübeck, and a few other cities. Discussions in the Bundestag and Bundesrat etc. might be conducted bilingually then.
Or this way:
The Hanseatic city states had been absorbed during a particularly weak period, becoming crown colonies. In the process of decolonialisation and Western European integration in the second half of the 20th century, they are granted independence, concluding loads of practically determined trade, defense and cooperation treaties with West Germany, and when the unification of Western and Eastern Germany comes along roughly around 1990, they join the FRG and a democratised GDR in a German Confederacy, who would, again, be bilingual out of respect for its few million fish-eating English speakers