Your issue is one of international civil process, which is usually a very complicated matter.
I suppose from your question that you have a trial before a state court of California, and the defendant is a US citizen with accounts in Germany, where it is hidden so as to avoid it being taken into account in your main trial.
The bank, itself is, usually not a party. In such a situation, the discovery of information regarding the bank accounts of the respondent must be done through the US court, via one of the following processes, or via voluntary cooperation:
1. Voluntary cooperation (for example, by formulating a written request sent to the head office of the bank, annexing the claim you have made to prove your legal interest);
2. The 1970 Hague Convention process, Germany being a contracting party to the Convention. The US court has to request evidence from the Central Authority in Germany through a 'letter of request'. The Central Authority in Germany transmits the letter to the competent judicial authority for execution. The letter must specify basic information, including the parties, the nature of the proceedings, a description of the evidence sought and, as appropriate, the specific documents or information requested to be disclosed. The letter must be translated into German before being sent. The Central Authority in Germany is obliged to respond and inform the US court of the developments, and send the gathered evidence;
3. Letters Rogatory, which is a slow and cumbersome process whereby the US court requests evidence from a foreign country via the US Department of State;
4. Subpoena to a US Citizen or Resident Abroad under the Walsh Act, 28 U.S.C. 1783. This is a very little used procedure which involves a determination that it is in the interest of justice that a person be served with a subpoena to produce documents from abroad.
I will point out that the German Central Authority is a misleading name, because there is one for each of the Lander in Germany, so you will need to know 1) the Land where the branch holding the accounts is located; or 2) address the request to the land of the bank's HQ. The list, including contact details, is available here: https://assets.hcch.net/upload/auth20de.pdf . There is also a useful guide for the court regarding the Convention in Germany here: https://www.hcch.net/en/states/authorities/details3/?aid=501
Also, I would like to encourage you to present the attached document to the court, which provides practical, up to date (it was released this week, updated) information on Evidence from abroad. Please check the bookmark regarding evidence from abroad, and also the Appendix regarding specific Germany information (pp. 68-70). The clerk or better your US attorney will know how to interpret better the provisions explained there. The key being that the letter of request / letter rogatory has to be expertly written so that the German authorities act as rapidly and effectively as possible.
Finally, two other pieces of information:
1. If the court requires it, you may point out that there is a semi-official translation of the full German Civil Procedure Code here: http://www.gesetze-im-internet.de/englisch_zpo/index.html
2. You may wish to consider hiring a private investigator (Detektiv) in Germany. They have the power to request information not normally available to other persons from institutions and private organizations. However, I advise you to be very specific in your request and confirm in advance that they are allowed and confident in obtaining the bank records, because they can be very expensive. I am not providing a list because as opposed to other countries there are many good English-speaking companies available on a simple search.
I hope my answer helps you and look forward to your rating.
Dr I L Vlad