PM Najib's lawyer Mohd Hafarizam Harun is reported to have said:
"The first issue you should look at is whether they have jurisdiction to take action against a prime minister of Malaysia, who is a Malaysian citizen and not the citizen of Switzerland, Germany or even the US.
"So can you (foreign investigators) extend your extra-territorial jurisdiction outside your country and bind a non-citizen?"
With the greatest respect to his learnedness, the "first issue" is going to be preventing the forfeiture and freezing of assets and bank accounts related to the investigation into 1 MDB and PM Najib Razak, which has already occurred in Singapore and Switzerland.
Then, given the investigations in the United States, it is useful to consider the powers of the US Justice Department and in particular the workings of its kleptocracy unit, as well as that of its Asset Forfeiture and Money Laundering Section (AFMLS). Taken together, these sections can seize assets in the US and foreign jurisdictions, and trace monies through the international banking system, imposing fines and sanctions against banks anywhere that deal in US dollars and which have correspondent banking arrangements with banks in the US. Banks which refuse to comply can therefore not only be faced with balance sheet breaking fines, they could also be frozen out of the international banking system. Consequently, while Malaysian banks may refuse to freeze or forfeit assets related to the 1 MDB scandal, they could find their businesses limited to Malaysian shores. No Malaysian bank can afford that type of isolation.
Appealing decisions of the US Department Of Justice will require petitions before courts in the United States ie submitting to US jurisdiction. Obviously , then, sovereign immunity cannot be raised.
With regards criminal proceedings against PM Najib personally, it is important to remember that sovereign immunity is usually recognized when the head of state or head of government is acting in his official capacity. Given all that has been said about donations to PM Najib personally, asserting now that the movement of billions of dollars into and out of Najib's personal accounts is a matter of state will not provide a shield. Asserting that Najib used it for party political purposes also will not work, no matter how strongly UMNO and BN feel about their ownership of the Government of Malaysia.The party is not the State.