Perhaps

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Note added at 2 hrs (2019-07-05 08:34:24 GMT)
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believes/claims/propounds BIS brain

It looks like Borio has been mentioned earlier in the article, and this is a reminder in case we've forgotten who he is. That's why his first name and job title are not given.

This is a common journalistic trick in English and German, though in English I think you can ignore "Denker" and just give his name and that of the organisation.

I would give his full name, "says the BIS's Claudio Borio."

I think "according to" would be a pretty standard phrase in journalism.
Somehow to me that phrase seems more natural at the start of a sentence:
According to BIS guru Borio, “The central banks must feel as though they had stepped through a mirror.”

It can also work at the end, though:
“The central banks must feel as though they had stepped through a mirror,” according to BIS guru Borio.

A slightly wordier option is "in the opinion of", which I think works well at the end.
“The central banks must feel as though they had stepped through a mirror,” in the opinion of BIS guru Borio.
And it's probably fractionally closer to "glaubt".

For "Denker", "Guru" is just one option, suggesting a person with much wisdom (and who probably spends a fair bit of time thinking).
Others could include:
* "wonk", suggesting much knowledge of (and interest in) technical/policy details;
* "oracle", suggesting the one person everybody turns to for deep, almost supernatural insight;
* "sage", suggesting someone a bit more studious or academic, perhaps;
* "expert" — a safe, generic option, albeit less connected to "Denker";
* ...

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Note added at 2 hrs (2019-07-05 08:48:39 GMT)
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Definitions of guru for the above context in point 2 at:
https://www.merriam-webster.com/dictionary/guru

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Note added at 2 days 6 hrs (2019-07-07 13:31:29 GMT) Post-grading
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Upon reflection, I think the possessive may work a little better in English, yielding
according to (the) BIS's guru, ... / in the opinion of (the) BIS's guru, ...

Note the obligatory addition of a comma ahead of the name, in this construction.

Note also the optional addition of the definite article ("the").

Some always use the definite article ahead of the abbreviation "BIS" ...
"Bringing the BIS to Asia - and Asia to the BIS" "In 2003, [...], the BIS formed the Asian Consultative Council."
https://www.bis.org/speeches/sp181018.htm

... some use it only in the article text (not the headlines) ...
"Passive Bond Funds Rewarding Debt Danger Stability, Says BIS" "In a report issued Sunday, the BIS warned that [...]."
https://www.bloomberg.com/news/articles/2018-03-11/passive-b...

... and some don't use it at all.
"BIS noted that [...]." "[...] before the publication of BIS’s report."
https://www.forexcrunch.com/bank-of-international-settlement...

The definite article is obligatory with the full name — even when not used with the abbreviation, as in the last link above.

(I also found one example using an erratic mixture within the article text, which is a mark of inconsistency rather than style: https://www.cryptoworldjournal.com/the-bis-concerned-over-fa... )

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Note added at 2 days 18 hrs (2019-07-08 01:31:17 GMT) Post-grading
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One more option for "Denker" is "analyst".
An analyst would implicitly be doing a lot of thinking about the matters under their consideration. A disadvantage is that "Analyst" has also become a job title (e.g., "Business Analyst"), which doesn't apply to Borio, and so it might be misconstrued.