Here’s Creifelds for a definition (the relevant sections of the StPO are given in English in the comment on February 12):

bq. Die Strafverfolgungsbehörden (StA, Polizei, Finanzamt usw.) haben nach §§ 152 II, 160, 163 StPO, § 386 AO bei Verdacht einer Straftat von Amts wegen, also auch ohne Anzeige, einzuschreiten. Dazu ist die StA befugt, von allen Behörden Auskunft zu verlangen und Ermittlungen jeder Art vorzunehmen. Ausnahmen gelten nach dem ®Opportunitätsprinzip insbes. in ®Bagatellstrafsachen. Das L. soll die Einhaltung des Grundsatzes der Gleichheit vor dem Gesetz (Art. 3 I GG) sichern. Es ist insofern die notwendige Ergänzung zum Anklagemonopol der StA, die im Einzelfall nach Abschluss des ®Ermittlungsverfahrens darüber entscheidet, ob das Strafverfahren durchgeführt werden soll …

Suggestions:
legality principle (this is definitely used sometimes but may be confusing)
principle of mandatory prosecution (too much emphasis on prosecution)
principle of prosecution ex officio
principle of mandatory investigation (too much emphasis on investigation? The duty goes further than investigating)

principle of discretionary prosecution
principle of prosecutorial discretion (I didn’t see the point of this when I first heard it, but prosecutorial avoids that problem of emphasis on the prosecution itself)

The terms used in practice are principle of mandatory / compulsory prosecution / legality and principle of expediency / opportunity principle. Here is a book on the criminal process by Ashworth and Redmayne, ina chapter on gatekeeping and diversion:

bq. In many European systems of law there is a doctrinal contrast between the principle of compulsory prosecution (sometimes called the principle of legality) and the principle of expediency (sometimes called the opportunity principle). Many systems, such as the German and the Austrian, have placed great emphasis on the principle of compulsory prosecution. …In theory all those who commit offences are brought before the courts for an open determination of guilt and (if convicted) for sentencing, and there is no broad discretionary power to avoid prosecution – on the grounds that this might lead to local variations …

Here is a nice online comparison of the German and U.S. criminal justice systems (PDF), published by the U.S. Department of Justice:

bq. English language works discussing German prosecutors generally focus on he German concept of obligatory prosecution (“legality principle”), a principle that contrasts sharply with the American emphasis on prosecutorial discretion (described by the Germans as the “opportunity principle”).

I reopened the comments on the February 12th entry, Questions on translating criminal law. That entry just collected some questions that had come up in a criminal law translation seminar in Frankfurt. Now I have received a further comment and posted it as anonymous (it came from a criminal court in Germany). But maybe the discussion should continue here.

Here are some queries from the comment: how to translate the following. I will come back to this in the next couple of days, I hope, when not dealing with Ludwig der Bärtige.

Anklageprinzip
Offizialprinzip
Ermittlungsgrundsatz
Grundsatz der Mündlichkeit
Grundsatz der Unmittelbarkeit?

Legalitätsprinzip: I objected slightly to mandatory prosecution because it sounds as if everyone had to be prosecuted.
Suggestion in comment: mandatory investigation, because there has been some erosion of the term.

yes, I agree, mandatory investigation sounds good. It was the word mandatory we were considering in the seminar, not prosecution. The reason I didn’t like mandatory prosecution was because it sounded as if the public prosecutor simply has to prosecute everybody. What it really means, I would say, is that the prosecutor has to prosecute in certain circumstances (although, as the comment illustrates, there has been some erosion of the term. Still, if it was originally so named because prosecution was mandatory (albeit only if the investigation produced evidence), why not keep the name now, in contrast to the less mandatory Opportunitätsprinzip?

Law com reports on a solo practitioner (BE: sole practitioner) attorney whose spellchecker replaced sua sponte with sea sponge.

bq. Spell-checking on his computer is never going to be the same for Santa Cruz solo practitioner Arthur Dudley.
In an opening brief to San Francisco’s 1st District Court of Appeal, a search-and-replace command by Dudley inexplicably inserted the words “sea sponge” instead of the legal term “sua sponte,” which is Latin for “on its own motion.”

Apparently the slip was noticed by the client (a former Santa Clara County Superior Court judge seeking reversal of his conviction for fixing traffic tickets report of decision here).

bq. The faux pas has made Dudley the butt of some mild ribbing around Santa Cruz. Local attorneys, he said, have started calling his unique defense the “sea sponge duty to instruct.”

In this connection, sua sponte might be von Amts wegen (also ex officio in English), but my Word wants to replace sponte with sponge.

I have encountered this kind of problem in texts to be translated before now.

Many thanks to Isabella, who has an eye for the important news.

No, I’m not wondering how to translate Ludwig der Bärtige – I’ve encountered him often enough before.

Or so I thought. But I was surprised to discover there were two Ludwigs der Bärtige. One was a count in Thuringia in the 11th century (more often Ludwig mit dem Barte?), and the other a duke in Bavaria-Ingolstadt in the 15th century. And good heavens, a third, a Wittelsbach born in 1901.

More cat content can’t harm: here you can apply for three of the numerous Bavarian cats that have been delivered to animal refuges in advance of the expected wave of avian flu. Otherwise just phone the refuges.
We are told on TV that some cat-owners are over-reacting. So they are, but on the other hand, does it help if at least half of the news programmes seem to be concerned with the topic? (Now interspersed with a certain amount of comment on the German soccer team)

I have encountered grober Vorsatz / grob vorsätzlich now for the second time. It’s definitely used, although I’m not sure it’s kosher (and I am not recommending its use). In my context it means Absicht.

|Vorsatz| intent(ion)|
|Arten des Vorsatzes| degrees of intention|
|Absicht (dolus directus ersten Grades)| concrete / specific intention|
|direkter Vorsatz (dolus directus zweiten Grades)| direct intent|
|bedingter Vorsatz (dolus eventualis)| conditional intent|

See section 276 of the German Civil Code. The terms occur mainly in criminal law, but my example was civil:

bq. …wenn der Vermieter den Mangel grob vorsätzlich oder grob fahrlässig zu vertreten hat…