The UK might have opened Pandora's box toward its decline.

English would not decline, but the UK might have opened Pandora's box toward its decline by the Brexit referendum. According to an article by a Japanese scholar, it will have to depend on economic relations with China, which is interested in its relationships with Germany as well as its relationships with the UK. I am not sure about this.
http://www.sankei.com/world/news/160627/wor1606270029-n1.html

P.S.
London and Frankfurt. I have been to London several times, but have never been to Frankfurt. I don't know if I have a chance to go to Germany before I grow too old to go abroad.

"Frankfurt is a global hub for commerce, culture, education, tourism and traffic. It is the most important financial centre of the European continent, with the HQs of the European Central Bank, German Federal Bank, Frankfurt Stock Exchange, Deutsche Bank, Commerzbank, DZ Bank, KfW, several fintech startups and other institutes."--Wikipedia