Fantastic question! In short, there isn't a correct answer to this, so the following is just my view on it. First, some clarifications:

1. Socialism in One Country isn't a stance against internationalism, but the belief that socialism can be achieved without revolution in the west. This dates back to Trotsky's insistence that the peasantry were counter-revolutionary due to having petty-bourgeois consciousness, and that the RSFSR would fall without support from a socialist west. The USSR practiced socialism in one country, but was directly interventionist, compared to the modern PRC.

2. The PRC's foreign policy during the sino-soviet split was awful. If you want to learn more about why, I gave my thoughts in a recent thread here. In short, they supported Cambodia over Vietnam, the US over the USSR, all because they thought that the USSR was an imperfect ally due to adopting a revisionist stance that class struggle was over in the USSR. They were right, and this revisionism contributed to their eventual collapse, but so did the split, especially with the terrible foreign policy on the PRC's part compared to the USSR's.

Alright, back to your questions!

Question: China is rapidly developing and practicing the “socialism in one country” policy, do you think they’d ever become internationalist like how Cuba or other states were?

I alluded to this already, but the PRC is already internationalist, just non-interventionist. I'll elaborate on that more later, but instead I'll answer what I predict will happen in the future:

As the US Empire continues to decline and the PRC continues to rise, there is a leftward turn within the youth of the PRC. Conditions are rapidly improving there, it's true, but staying so interconnected with the global market is a calculated risk with consequences for the working classes as well. They've relied heavily on exports, and this made them reliant on the world's biggest treatlerites, the US Empire.

In the latest Five Year Plan, however, the one for 2026-2030, a huge focus is being placed on raising domestic consumption, lowering working hours, and raising quality of life directly. The last few Five Year Plans have been focusing on green development, rapidly improving production, and equalizing the rural/urban divide that was sharp due to rapid development in the cities, but now that those are well under-way, China is beginning to try to rely on itself moving forward.

This opens them up to more interventionism, as the multi-polar world emerges, if they so choose, such as if the US Empire truly does try to spark a hot war in Taiwan/Japan/ROK (though the ROK is moving more towards the PRC these days and against Japan/US).

If so, what are the requirements?

Essentially, they need to not depend on the US Empire for exports, and drive up domestic consumption, something they've already aconowledged. The PRC also has a defensive millitary, not an imperialist one like the US Empire, so they'd need to pivot their range to a more active role, something the US is trying to prevent now by couping a bunch of states in South America.

And why are they not currently?

This is where I will answer how they are already internationalist. It's because right now they are undermining the basis of imperialism by focusing on win-win development with the global south, and bypassing unequal exchange. A huge part of how unequal exchange functions is tech monopoly allowing the west to charge monopoly prices for tech, but China doesn't do that. That's why BRI and cheap EVs, solar, etc. have been seeing a huge swing in the global south, and has allowed the global south to escape underdevelopment.

It's a boring, slow, gradual internationalism, but they are trying to build that multi-polar world where the US Empire isn't the only player. That's why the US Empire is increasingly desperate to stop China, when they seemed just fine only 15 years ago. Chinese foreign policy during the Sino-Soviet split was terrible, and this is also a course-correction to not directly make the same mistakes they did for the latter half of the 20th century.

As time goes on, though, the youth are more radical and aren't simply content with the current level of development, they dream of the social safety nets of the Maoist era and yearn more for direct action. The process of democracy in the PRC is slow and gradual, but does respond to the will of the people. As these demographics shift, and more youth enter the CPC, we will likely see a more radical shift.

Just as Mao, Deng, and Xi all adapted to their present material conditions of China, so too will Xi's successor have to, and considering 2049 is the 100th anniversary of the founding of the PRC, expect big shifts in the years leading up to that.

Sorry that this was so long!