山上五老虎, 猴子称大王

山上无老虎 shan1shang4wu2lao3hu3
猴子称大王 hou2zi5 cheng1 da4wang2

"[On the] mountain high [there is] no tiger,

[the] monkey declares [himself the] great king." (literal translation) [brackets are added words]

This Chinese proverb (cheng yu) translates well as "when the rats away the mice will play".  I have no view on translation theory whether using the similar proverb is the better translation though less literal. I tend to prefer translating as closely as possible to the original text, which is especially necessary in legal translations.

Politically, it may refer to the idea that Angela Merkel is nearly fully absent now; or, atlernatively, that President Biden is too old to adequately govern.

The question we must ask then is who is the monkey?

This proverb may also refer to an inner-party struggle. I would hazard the guess then the monkey might be 胡锦涛. I don't think that really makes sense but maybe?
It's probably about Biden.