I would say: letter of employment

A testimonial usually consists of positive comments about a product or service.

In Canada, possibly also in the US (not sure), letters of referral, certainly letters of recommendation, are rarely given these days AFAIK.

I worked in the corporate world for a long time. Letters of referral/recommendation were discontinued a long time ago due to the possibility of error. (i.e., Someone may have been a fair or even great employee for you, but fail miserably in their new job .... ).

The letters given by employers now usually consist of: XYZ worked for ABC corporation from xx/xx/xx to xx/xx/xx. Period.

Was wäre die passende ... Überschrift für so ein Schreiben, das im internationalen Kontext korrekt verstanden wird?

"DO NOT READ" 🙂

In addition to #2: The original German version of this document is filled with code words and stock phrases that German HR departments use to make even a lousy reference sound positive. While other German HR departments can easily decipher them, an English-speaking reader most likely cannot, and the translation may also not reflect the nuances of the German.

#0: "letter of recommendation" - ich verstehe darunter etwas ganz anderes. (Führe das aber mal nicht hier aus.)

#4, iskreamtoffee, nebenbei:

Ich sehe etliche Rechtschreibfehler und fehlende Kommas ... Bitte vor 'dass' immer ein Komma setzen. Leerzeichen vor und hinter einen Gedankenstrich. (Wohlgemerkt: bei Gedankenstrichen, nicht Bindestrichen.) Da LEO ein bekanntes Sprachforum ist, bitte ein bisserl Mühe geben, danke :- )

I speak BE.

I think it depends on what the document contains. It could range from "certificate of employment", which might only provide basic details such as job title, starting and finishing date, to "employment reference", providing some information about duties and, perhaps, performance to "letter of recommendation", which would be very positive (although rather old fashioned).

"Employment reference" would be a good catch-all.