Gemara

(Beraisa - Sumchus): If one said 'I am a Nazir...' and concluded 'Hen', he is a Nazir once. If he concluded 'Digun', 'Trigun', 'Tatrigun', or 'Penitgun', he is a Nazir two, three, four or five times, respectively.

Nedarim 10a (Mishnah): Nazik, Nazi'ach, and Pazi'ach are Kinuyim (substitute words) for Nezirus.

(R. Yochanan): Kinuyim are from Nochri languages;

(Reish Lakish): Chachamim concocted them for people to use, lest one say "la'Shem Korban", and come to say only "la'Shem", Hash-m's name in vain!

The Tana (of the Mishnah 2a) taught Kinuyim before Yados because Kinuyim are mid'Oraisa.

Question: This is like the opinion that Kinuyim are from Nochri languages. According to the opinion that Chachamim invented Kinuyim (also they are mid'Rabanan), how can we answer?

Rishonim

The Rif and Rosh (Nedarim 2b and 1:8) bring the Mishnah of Kinuyei Nezirus.

Rambam (Hilchos Nezirus 1:8): All Kinuyim for Nezirus are like Nezirus. E.g. in places where people garble words and one said 'I am Nazik, Nazi'ach, or Pazi'ach', he is a Nazir.

Radvaz (Hilchos Nedarim 1:16): It is not clear if the Rambam rules like R. Yochanan or Reish Lakish. Presumably, he follows R. Yochanan against Reish Lakish. However, he found it difficult why Nochri languages should work. Rather, it is because there are people who speak improperly. Therefore, it applies even where there are no Nochrim, or where they have no word for Korban. We follow the substitute name because Nedarim follow the way people speak. It does not matter whether or not the one who vowed can speak properly.

Question: R. Yochanan holds that Kinuyim are from Nochri languages. If so, why does our Mishnah bring these specific examples? It should say that a Neder in any language is valid!

Answer #1 (Ran 2a DH Kol, citing R. Yehudah b'Ribi Chisdai): The Mishnah teaches a bigger Chidush. Not only proper translations (of Neder, Nezirus...) in other languages are valid. Rather, even these improper expressions, which came from Lashon ha'Kodesh but were mangled, are valid. One might have thought that they are invalid, for they are not proper speech. The Mishnah teaches that they are valid, for Nochrim use them. All the more so, other languages work. The Yerushalmi confirms this explanation.

Answer #2 (Ri, in Tosfos 2a DH Kol): If one vowed with one of the Kinuyim in the Mishnah, it is valid even if he did not understand it. Any other Kinuy works only if he understood it.

Question (Rashba Nazir 2a DH Masnisin, Me'iri Nazir 3a DH Kevar, Tosfos Yom Tov Nazir 2:1 DH Nazik): How can it take effect without understanding? His mouth and heart must be united (Terumos 3:8)!

Answer (Orach Mishur on Tosfos Nazir 2a DH Nazir): Tosfos must hold like those who say that Devarim sheb'Lev count only when he did not say the words he wanted to say. If one said the words he wanted, but intended that it not take effect, it takes effect (Mordechai, Tosfos Shevuos 26b DH Gamar, and Shulchan Aruch YD 211:1).

Question (Rashba, ibid.): The vow takes effect only if he intended to vow, whether or not he understood the words he said. There is no difference between Leshonos (languages, or the words used)!

Answer (Kovetz Shi'urim Bava Basra 597): If he vowed in Lashon ha'Kodesh, since everyone knows it, if he does not this is Devarim sheb'Lev.

Answer #3 (Rashba, ibid.): The Tana teaches that these Kinuyim work, but Kinuy Kinuyim, e.g. Mekanama (in place of Konam) do not work.

Question: Tosfos (ibid.): According to the opinion that Kinuyim are mid'Rabanan, how can a Nazir bring Korbanos for Nezirus mid'Rabanan? It is not difficult how one is liable for Bal Te'acher for a Neder mid'Rabanan.

Answer #1 (Tosfos): Indeed, all agree that Kinuyei Nezirus are from Nochri languages. They argue only about Kinuyim for Nedarim. This is why the Gemara in Nazir (2b) did not ask according to Reish Lakish why the Mishnah (Nazir 2a) begins with Kinuyim.

Rejection (and Answer #2 - Ri, in Tosfos, ibid.): Often, the Gemara asks a question in one place, and not elsewhere! Rather, Chachamim have the power to make their enactment like Torah Nezirus.

Answer #3 (Ri, ibid., and Tosfos Nazir 2a DH Nazir): Since he wants to accept Nezirus, and he knows that Chachamim concocted this expression for Nezirus, it is a full acceptance.

Answer #4 (R. Yechiel in Tosfos, ibid.): Reish Lakish did not mean that he brings Korbanos for Nezirus through a Kinuy. Rather, he is lashed if he transgresses Nezirus.

Answer #5 (Orach Mishur ibid., b'Sof): Reish Lakish holds that the entire Isur of offering Chulin in the Mikdash is mid'Rabanan (Nazir 29b).

Poskim

Rema (YD 207:1): If one does not know the expressions that Chachamim spoke about, even if he vowed by them, it is void.

Gra (3): This is because his mouth and heart are divided. This is unlike Tosfos (Nedarim 2a).

R. Akiva Eiger: If he wanted to vow with the Kinuy, it is valid, even if he does not understand what it means. His mouth and heart are equal! The Rashba (Reish Nedarim) agrees.

Shulchan Aruch (ibid.): Kinuyei Kinuyim, i.e. a Kinuy that is very far from the proper word, is invalid.

Source (Gra 2): R. Yochanan holds that Kinuyim are from Nochri languages.