Sinim

And I will make all my mountains a way, and my highways shall be exalted. Behold, these shall come from far: and, lo, these from the north and from the west; and these from the land of Sinim.

Source: wiktionary

And then, O house of Israel, behold, these shall come from far; and lo, these from the north and from the west; and these from the land of Sinim.

Source: wiktionary

The opinions of commentators are equally varied; some, as Jerome, Jarchi, Grotius, Pfeiffer, and Forerius, suppose that by the land of Sinim is meant the peninsula of Arabia, and particularly the Desert of Sin, and the region around Mount Sinai. Others prefer to understand Egypt, two of whose cities are called, in Scripture, Sin and Syene. Some of the most respectable names among commentators uphold this opinion; among others, those of the Jewish writers Aben Ezra, and Kimchi; and of the Christians, Bochart, Vitringa, Hiller, Seeker, Munster, Clarius, Michaelis, Orton, W. Lowth and Thomas Scott. There is another class composed of those who think that the land of Sinim means China, among whom are Manasseh Ben Israel, Arias Montanus, Dorsch, Langles, Gesenius, Calmet, Dr. Hagar, Dr. Morrison and others. Probably the truth lies between these opinions, and to them our attention may be confined.

Source: wiktionary

From the internal evidence offered by the context, the country of Sinim is a far distant land, which must be sought for in the East, the only point of the horizon left unnoticed by the author. The word Sinim exhibits the grammatical form of the Semitic plural, therefore it is an ethnic term—the name of a people, not that of a country. In the Septuagint version the difficulty of identification, ipsis verbis, was explained away by a simple substitution of names. The country of the Persians ἐκ γῆς Περσῶν, was named instead of the country of Sinim.

Source: wiktionary