A civilization on a single island and with constant resources would naturally have nearly constant population. In both ancient and medieval times this was the common case. People simply did not start families and have children unless they could afford them. And since the resources would be limited so would the population.
Unity of culture and language
If travel is difficult due to physical obstructions the population will fragment to multiple separate sub-populations with their own, probably related cultures, religions, and languages.
I'd assume that with an island most of the population lives near the coasts or areas well connected to the coast and is well connected by sea transport. In this case there would probably be single culture and language. Details vary with social development. Without developed economy there is little reason for population centers to trade or unite and the islands are dominated with small chieftaindoms with their own dialects and customs.
For your scenario we can probably assume a developed economy and significant trade and travel leading to unified culture and language. There would still be regional dialects among the common people, but the educated people would probably make an effort to avoid rustic accent and uneducated language. Low population inland areas might have tribal languages and cultures. These would still be heavily influenced by the main culture.
Rate of development for technology
Rate of development is roughly related to the amount of "relevant" people in contact. If we assume a single culture, the connectivity is fairly good. Similarly a stable environment would have given lots of time for specialization. There would be stable traditions of most crafts and arts.
The two remaining variables are basically related to the food supply. If food requires little work to produce, more of population is available for specialized production. If food is abundant, higher population density can be supported, a smaller island is needed and travel times between population centers are reduced. None of these variables is known. I'd assume an economy based fishing, farming, and pastoralism.
Fish on a world with single habitable island with restricted population would presumably be abundant. A lot depends on the location of the island, but if we assume it is the only island around, we can assume it makes the ocean currents do interesting things that raise nutrients to the surface and create a fishing bonanza. So fish would be easy to get, abundant, and fairly portable. It would also supply a diet rich in protein.
Agriculture would be limited by the amount of arable land and might be vulnerable to erosion and drought. Unless we want an impending ecological catastrophe, we can assume this has been dealt with and the agricultural land has decent irrigation and erosion control with fertilization provided from waste from fishing and domestic waste. Such agriculture should be fairly productive.
These assumptions would support a society with fairly high level of specialization and large cities. As such the culture would be highly developed and have a level of technology similar to middle ages but with high level of sophistication. Somewhere along the Roman and Chinese empires combined and with more polish, maybe.
The basic issue is that with extremely stable, even stagnant, economy and sharply restricted available resources there is nothing to really drive the mass production or industrialization. You can assume a pandemic similar to the Black Death breaking the status quo and forcing the society to use labor more efficiently. This would push the society to the renaissance level with more sophisticated economy and production. But as their is no real space for economic expansion the society would probably revert back to its stable state.
