From the Writings of Baha’u’llah:
Among the things which are conducive to unity and concord and will cause the whole earth to be regarded as one country is that the divers languages be reduced to one language and in like manner the scripts used in the world be confined to a single script. It is incumbent upon all nations to appoint some men of understanding and erudition to convene a gathering and through joint consultation choose one language from among the varied existing languages, or create a new one, to be taught to the children in all the schools of the world.
(Baha’u’llah: Tablets of Baha’u’llah, pp. 165-166)

It behoveth the sovereigns of the world – may God assist them – or the ministers of the earth to take counsel together and to adopt one of the existing languages or a new one to be taught to children in schools throughout the world, and likewise one script. Thus the whole earth will come to be regarded as one country.
(Baha’u’llah: Tablets of Baha’u’llah, p. 22)

From the beginning of time the light of unity hath shed its divine radiance upon the world, and the greatest means for the promotion of that unity is for the peoples of the world to understand one another’s writing and speech. In former Epistles We have enjoined upon the Trustees of the House of Justice either to choose one language from among those now existing or to adopt a new one, and in like manner to select a common script, both of which should be taught in all the schools of the world.
(Baha’u’llah: Tablets of Baha’u’llah, p. 127)

The day is approaching when all the peoples of the world will have adopted one universal language and one common script. When this is achieved, to whatsoever city a man may journey, it shall be as if he were entering his own home.
(Baha’u’llah: Gleanings, pp. 249-250)

O members of parliaments throughout the world! Select ye a single language for the use of all on earth, and adopt ye likewise a common script.
(Baha’u’llah: The Kitab-i-Aqdas, p. 88)

From the Utterances of `Abdu’l-Baha:
Baha’u’llah has proclaimed the adoption of a universal language. A language shall be agreed upon by which unity will be established in the world. Each person will require training in two languages: his native tongue and the universal auxiliary form of speech. This will facilitate intercommunication and dispel the misunderstandings which the barriers of language have occasioned in the world.
(`Abdu’l-Baha: Promulgation of Universal Peace, p. 300)

One of the great steps towards universal peace would be the establishment of a universal language. Baha’u’llah commands that the servants of humanity should meet together, and either choose a language which now exists, or form a new one. This was revealed in the Kitab-i-Aqdas forty years ago. It is there pointed out that the question of diversity of tongues is a very difficult one. There are more than eight hundred languages in the world, and no person could acquire them all.
The races of mankind are not isolated as in former days. Now, in order to be in close relationship with all countries it is necessary to be able to speak their tongues.
A universal language would make intercourse possible with every nation. Thus it would be needful to know two languages only, the mother tongue and the universal speech. The latter would enable a man to communicate with any and every man in the world!
(`Abdu’l-Baha: Paris Talks, pp. 155-156)

Today the greatest need of the world of humanity is discontinuance of the existing misunderstandings among nations. This can be accomplished through the unity of language. Unless the unity of languages is realized, the Most Great Peace and the oneness of the human world cannot be effectively organized and established because the function of language is to portray the mysteries and secrets of human hearts. The heart is like a box, and language is the key. Only by using the key can we open the box and observe the gems it contains. Therefore, the question of an auxiliary international tongue has the utmost importance. Through this means international education and training become possible; the evidence and history of the past can be acquired. The spread of the known facts of the human world depends upon language. The explanation of divine teachings can only be through this medium. As long as diversity of tongues and lack of comprehension of other languages continue, these glorious aims cannot be realized. Therefore, the very first service to the world of man is to establish this auxiliary international means of communication. It will become the cause of the tranquillity of the human commonwealth. Through it sciences and arts will be spread among the nations, and it will prove to be the means of the progress and development of all races. We must endeavor with all our powers to establish this international auxiliary language throughout the world. It is my hope that it may be perfected through the bounties of God and that intelligent men may be selected from the various countries of the world to organize an international congress whose chief aim will be the promotion of this universal medium of speech.
(`Abdu’l-Baha: Promulgation of Universal Peace, pp. 60-61)

Diversity of languages has been a fruitful cause of discord. The function of language is to convey the thought and purpose of one to another. Therefore, it matters not what language man speaks or employs. Sixty years ago Baha’u’llah advocated one language as the greatest means of unity and the basis of international conference. He wrote to the kings and rulers of the various nations, recommending that one language should be sanctioned and adopted by all governments. According to this each nation should acquire the universal language in addition to its native tongue. The world would then be in close communication, consultation would become general, and dissensions due to diversity of speech would be removed.
(`Abdu’l-Baha: Promulgation of Universal Peace, pp. 232-233)

Ninth, a universal language shall be adopted and be taught by all the schools and institutions of the world. A committee appointed by national bodies of learning shall select a suitable language to be used as a medium of international communication. All must acquire it. This is one of the great factors in the unification of man.
(`Abdu’l-Baha: Promulgation of Universal Peace, p. 182)

From the Writings of the Universal House of Justice:
Baha’u’llah enjoins the adoption of a universal language and script. His Writings envisage two stages in this process. The first stage is to consist of the selection of an existing language or an invented one which would then be taught in all the schools of the world as an auxiliary to the mother tongues. The governments of the world through their parliaments are called upon to effect this momentous enactment. The second stage, in the distant future, would be the eventual adoption of one single language and common script for all on earth.
(The Universal House of Justice: Aqdas: Notes, p. 250)