Here's a video explaining what was the first language, and I agree with him.
http://www.youtube.com/watch?v=XqacOagIhJI
Several years ago, a guy wrote about why Hmong has two dialects:
"In the beginning, there was only one group of Mong, but when the Mong and the Chinese fought against each other, the Mong were scattered and divided into many groups. Since then, the Mong began to speak different languages and form various groups, such as Mong Dlu, Mong Yao Chua and other Mong that may be unknown to us today. Nevertheless, the articulation of the term “Mong� is still the best pronunciation for everyone. The pronunciation of the word "Hmong" is just for one group that had outshooted or transformed from those speaking the Mong language. The term "Mong" is the original or proto root word for the Mong language that the Mong Leng still use today.
The truth behind the “Mong Leng� and the “Hmong Der,� is that they are one group of people. Everyone was united with one goal in mind, which was to protect the name "Mong." Everyone fought against the same enemy from era to era regardless of the wars they were involved in.
Due to the wars, the term Mong evolved into two names or two distinguishable groups of people. A Mong group transformed itself and changed their language. After this transformation, they called themselves "Hmong" and stated that they were "Hmong Der", meaning the innocent one.
The cause for having the name "Hmong Der" was not due to conflicts or problems within the Mong. It was a means devised by a Mong female to save one group of Mong only. She sought ways to save the men of her group. During that time, the Mong and the Chinese fought a major battle. When the Mong lost the battle, the Chinese went after them. The Chinese marked all the Mong men for extinction. The sister of this group came up with an idea that "they needed to change their language and costumes to make them different from the proto-Mong; in that way, she could justify to the Chinese that her group was the innocent one. They looked different from the Mong that fought against the Chinese." Therefore, they changed their costumes and language into something different. From that time on, this group of Mong began to wear white skirts and pants with sashes when they dressed. The men changed from wearing full-length pants and extended crotches to fitted pants with short crotches. This group of Mong began to speak the language called Hmong Der and use the term “Hmong� as we know today. The truth behind this story is firmly supported, evidenced by a funeral ritual still practiced by the Hmong Der today. The Hmong Der still highly regarded their "sisters," sometimes referred to "aunts" with great respect. When a brother dies, the sister plays an important role in his funeral ceremony. It is because at one time in history a sister had saved their ancestors from Chinese persecution. When there is a misfortune in their families, Hmong Der will call on their sisters or their aunts to dwarf off or drive away the misfortune, so that the misfortune will not occur and do harm to their future generation or their descendents.
As for the Mong who have kept their original or proto language, their name has come to be known as “Mong Leng�. It was not that they had "leng" [meaning they have "veins," but the word “leng� in Mong means “admitting�] so they called themselves Mong Leng. They were the ones who had to admit that they had fought against the Chinese. They were the majority who faced the consequences and who were persecuted very harshly by the Chinese during this period because they hold on to their name, their language and culture, and their costumes. This group did not have a sister or an aunt to save them at that crucial time. Since then, they have called themselves “Mong Leng� because they admitted to the Chinese that they were the ones who had fought against the Chinese. Today, Mong Leng still hold on to the term “Mong.� They do not practice the funeral rituals like the Hmong Der who have regarded their sisters or their aunts highly during the funeral ceremonies.
Even though Hmong Der had escaped persecution that time, the Chinese never ceased to persecute the Mong as a people. Despite their different languages and different costumes, both groups remained committed to fight their enemy and helped each other until this day. The Mong had fought many wars with the Chinese. One time, the “Three Mong� [groups] combined their forces to fight against the Chinese, so the term “Three Mong� emerged, and later became the term “We Mong� [because “peb� also means “we�] that still in use today.
There are Mong living in China, Myanmar (Burma), Thailand, Laos, Vietnam, and other western countries. We have both Mong Leng and Hmong Der. One group may not have more population than the other, but when comparing all the Mong population combined in the world, the Mong who speak Mong Leng are the majority.
This story is not written to compliment or offend anyone, but it is written to dispel the truth behind the two names “Mong� and “Hmong.� This story is not coming out of the blue, but it is written to record the story that has been retold time after time by our ancestors who have passed on this story to us from grandparents to grandchildren and from generation to generation until today.
Such a story as this one, if not preserved and written down, it may disappear. Given this reason, I decided to write this story down and share it with you. If there is anyone who knows any story about the Mong people, I encourage you to write and share it with us so that we can all learn from our past history."