Rumi did not just write in Persian
- He spoke in Persian &
- all of his sermons ("Fihi ma Fihi" book), recorded by his students in real time, are in Persian
- The "Fihi Ma Fihi" is in an informal Persian and is best proof that Rumi spoke Persian in everyday affairs
- Had he spoke Turkish in everyday affairs, then his lectures would have been recorded in Turkish
- But they are in informal Persian
- that puts an end to the theory that "he wrote Persian because it was more beautiful"
- Obviously, the proponents of this theory have not read:
--- neither "Fihi ma Fihi", nor the "Seven Sermons" (Friday sermons, spoken & documented in Persian)
- Also Rumi's recorded conversations with Shams are in Persian (though, some are in Arabic), but never Turkish
So we have:
A. His conversations with his students - all in Persian.
B. His friday sermons - all in Persian.
C. His conversations with Shams, primarily and overwhelmingly - in Persian.
As per the verse you claim, Rumi also says:
"To Maah Torki o man Agar Tork nistam -
daanam beh in qadar keh beh Torkist, ab su"
تو ماه ِ ترکي و من اگر ترک نيستم
دانم من اين قَدَر که به ترکي است، آب سُو
“You are a Turkish moon, and I, although I am not a Turk,
know that much, that in Turkish the word for water is su”
-- A-M. Schimmel, "Triumphal Sun", p. 196
Turkish & Hindu have very different (mainly symbolic) meaning in Persian poetry and show opposites.
Here is another example:
گه تركم و گه هندو گه رومی و گه زنگی از نقش تو است ای جان اقرارم و انكارم
Gah torkam, Gah Hendu, Gah Rumi, Gah Zangi
Az Naqsh tost ay del, Eqraaram o Enkaaram.
“I am sometimes Turk, sometimes Hindu, sometimes Rumi, and sometimes Negro,
O Soul, from your image is my approval and denial”
So these imageries are not a proof of background:
- Turk vs Hindu
- Rumi vs Black
are favorite symbols of Persian poetry.
The verse you brought says: "Agarcheh hendu gooyam"
- Assuming authentic, it means: "I speak in Hindu"
- but, now we know Rumi did not speak in Hindu.
- however: Turk vs Hindu, is a contrast of climates, colors, lifestyle, kings vs desolates & etc in Persian poetry.
The connection of Rumi's mother to the Khwarizm-shah
- is seen as legendary hagiography &
- not factual, due to both chronological reasons, as well as textual reasons
- modern scholars reject it & it was designed to simply connect him to royalty
- In reality, the grandmother of Rumi is a simple woman, as demonstrated by Baha al-Din's "Maarif"
- I would read the articles in Encyclopedia of Islam on Rumi, as well as the book of Franklin Lewis
- There are also the secondary sources acceptable in Wikipedia.
Although Hindu, Turk, Rumi (Roman) and Black are favorite symbols of Persian poetry
- and even "Rumi" is called Rumi
- if you look at Aflaki, there are also some comments about Turks
There is an anecdote from Rumi, quoted by Aflaki (pg. 503), about Tukrs vs Greeks
Note, he is not putting Turks downs or praising Greeks (in my opinion),
but just making an observation:
- "Likewise, it is a well-known story that one day Shaykh Salah al-Din happened to hire
- Turkish laborers to do building work in his garden
- ‘Effendi’— that is to say lord — Salah al-Din,
- when it is time for building, one must engage Greek laborers and
- when it is time for destroying something, Turkish hirelings.
- the building of the world is assigned to the Greeks,
- whereas the world’s destruction is reserved for the Turks.
- When God — He is sublime and exalted — ordered the creation of the world of sovereignty (‘alam-e molk’):
- first, He created unaware - infidels, and
- He conferred on them long life and great strength,
- so they would strive like hired laborers in building the terrestrial world.
- And they built up many cities and fortresses on mountain peaks and places on top of a hill,
- such that after generations had passed these constructions were a model for those, who came later.
- Then divine predestination saw to it that little by little
- these constructions would become completely destroyed, and desolate, and be eradicated.
- God created the group of Turks, so that they would destroy
- every building they saw, mercilessly and ruthlessly, and cause it to be demolished
- And they are still doing so, and day by day until the Resurrection they will continue to destroy in this manner.
- In the end, the destruction of the city of Konya will also be at the hands of wicked Turks devoid of mercy.’
- And this being the case, it turned out just as Mowlana said"</i>
See: Shams al-Din Ahmad al-Aflaki, "Manāqeb al-ārefīn", ed. Tahsin Yazici, 2 vols, Tehran, Donyaayeh Ketab, 1983, p. 503.
English translation: Shams al-Din Aflaki, "The feats of the knowers of God: Manāqeb al-ʻārefīn", translated by John O'Kane, Brill, 2002.
Here are some quotes from Rumi:
- "God created the group of Turks, so that they would destroy every building they saw, mercilessly and ruthlessly, and cause it to be demolished."
- "Oh ignorant Turk! Give up (tark) this idea and undertaking. Take back your Turks (torkan) to your lady (tarkan) as quickly as possible. Otherwise, you will not escape with your life"
- "Majd al-Din, why did you let out a shout and release your quarry from your gullet? A Turk who is a recent disciple is able to bear the burden, but you divulge the matter. Many things like this occur to abdals to God.”
- "Indeed, the building of the world is assigned to the Greeks, whereas the world’s destruction is reserved for the Turks.“
Note, that I only brought these for demonstrations:
- The "Diwan-e Shams" overall contains a positive usage of the word Turk,
- the Mathnawi seems somewhat neutral towards negative,
- but the "Manaqib" by Aflaki seems negative
- The reason is that "Diwan-e Shams" is a mystical book &
- the imagery of Turk in Persian mysticism has been positive
- along with that of "Rum/Rumi" = Greeks.
Rumi's son on multiple occassions has attested that his Turkish is very poor
- Yet his son was born in Anatolia, but he claims little knowledge of Greek and Turkish
- This is described in this article 
- According to Franklin Lewis:
--- “Sultan Valad elsewhere admits that he has little knowledge of Turkish”
--- Franklin Lewis, "Rumi Past and Present, East and West", Oneworld Publications, 2000, p. 239
--- “Sultan Valad did not feel confident about his command of Turkish”
--- Franklin Lewis, "Rumi Past and Present, East and West", Oneworld Publications, 2000, p. 240
- Sultan Walad actually admits the fact that his knowledge of Turkish & Greek is rudimentary 4 times
--- e.g. in the "Ibtedanama", Sultan Walad states:
--- بگذر از گفت ترکی و رومی که از این اصطلاح محرومی گوی از پارسی و از تازی که در این هر دوخوش همیتازی
"Abandon the speech of Turkish and Greek
Since you are deprived of these expressions.
Instead speak Persian and Arabic
Because you are well versed in these two.
--- Sultan Walad, "Masnaviyeh Waladi", Ensha’ Baha al-Din b. Mowlana Jalal al-Din Mohammad b. Hosayn-e Balkhi, Mashur beh Mowalana, ed. Jalal al-Din Homai (Tehran: Eqbal, 1316) (pp. 393-4)
--- Even Sultan Walad's son admits 3-4 times that he has very poor command of Greek and Turkish.
A complete response to the arguments you have and could have is given here 
- Wikipedia works by standards of Western scholars
- A-M. Schimmel and Franklin Lewis are the top Rumi scholars &
- they have called Rumi a Persian man & a Persian poet
- That is sufficient
- It is unfortunate that the same arguments get repeated again and again
- It is extremely tiring that instead of reading the archives,
- some new user always comes to make the same repeatative arguments
- The fact is Rumi is known because of his Persian poetry.
- No one is going to examine his corpse for DNA evidence.
Wikipedia works by WP
- weight and WP
- Western scholars in general & Rumi Western scholars in particular (like Franklin Lewis & A-M. Schimmel)
- affirm Rumi's heritage & cultural background as Persian
- So that is what counts & not endless polemics that is constantly repeated.
Encyclopedia of Islam is another weighty source
1. Franklin Lewis, "Rumi Past and Present, East and West", Oneworld Publications, 2000, pg. 9
"How is it that a Persian boy born almost 8 hundred years ago in Khorasan, the northeastern province of greater Iran, in a region that we identify today as Central Asia, but was considered in those days as part of the greater Persian cultural sphere, wound up in Central Anatolia on the receding edge of the Byzantine cultural sphere, in which is now Turkey, some 1500 miles to the west?"
2. Annemarie Schimmel, “The Mystery of Numbers”, Oxford University Press,1993. Pg. 49:
“A beautiful symbol of the duality, that appears through creation, was invented by the great Persian mystical poet Jalal al-Din Rumi,
who compares God's creative word 'kun' (written in Arabic KN) with a twisted rope of 2 threads (which in English twine, in German Zwirn, both words derived from the root “two”)”
3. Ritter, H.; Bausani, A., "Jalāl al-Dīn Rūmī b. Bahā al-Dīn Sulṭān al-ulamā Walad b. Ḥusayn b. Aḥmad Ḵhaṭībī" in "Encyclopaedia of Islam", Edited by P. Bearman, Th. Bianquis, C.E. Bosworth, E. van Donzel and W.P. Heinrichs. Brill, 2007. Brill Online. Excerpt:
"known by the sobriquet Mawlānā (Mevlânâ), Persian poet and founder of the Mawlawiyya order of dervishes"
4. Julia Scott Meisami, Forward to Franklin Lewis, "Rumi Past and Present, East and West", Oneworld Publications, 2008 (revised edition)
5. John Renard, "Historical dictionary of Sufism", Rowman & Littlefield, 2005, pg. 155:
"Perhaps the most famous Sufi, who is known to many Muslims even today by his title alone -
is the 7th/13th century Persian mystic Rumi"
6. Frederick Hadland Davis, "The Persian Mystics. Jalálu'd-Dín Rúmí", Adamant Media Corporation, 2005, ISBN-10: 1402157681
If you are interested in this matter, read here 
- However from Wikipedia's point of view, the most comprehensive books on Rumi are those written by Rumi scholars
- amongst them, the book of Franklin Lewis currently stands out as the most detailed & objective biography of Rumi in any language.
- And he is a Professor of University of Chicago