What ethical issues do lawyers have to think about when representing clients in cross-border disputes? How do these issues change depending on the country and culture of the dispute?
There are various types and levels of ethical considerations that a lawyer should consider while handling cross border cases. Some of them include:

1) Nationality and citizenship laws – A lawyer needs to be familiar with local law regarding nationality, immigration or asylum policies, especially if it may impact how they represent their client's case. This can also apply for other legal jurisdictions like tax matters. For example, US citizens who move abroad need to comply with the new jurisdiction's tax laws before filing taxes there. The same goes for foreign nationals living in countries where taxation is based on residency rather than citizenship status such as Germany.
2) Privacy Laws - Depending on which state/country your client resides in you might encounter privacy restrictions related to data protection laws. These regulations usually prohibit the use of personal information without explicit consent from individuals involved. In certain circumstances even international organizations (e.g., UN agencies, EU offices), might fall under this umbrella.
3) International Trade Agreements - Cross border transactions often involve trade agreements such as WTO or FTAs between multiple nations. If one party does not abide by the terms of an agreement then the transaction will likely get blocked at customs due to noncompliance. Therefore it would be prudent to ensure compliance before proceeding further into negotiations so as to avoid any future complications.
4) Foreign Corrupt Practices Act - Many governments impose sanctions against companies engaged in bribery practices overseas including the USA. As a professional lawyer, you must know what actions you take could trigger FCPA violations and make you vulnerable to penalties.