Feast - Language at

As a general principle, the Nineteen Day Feast and other official Bahá’í gatherings should be conducted in the conventional language spoken by the people of the locality. However, as social and economic conditions throughout the world continue to change, it is not unreasonable to assume that more and more people will be forced to migrate to urban centres, forming pockets of minorities, each with a distinct language, as can already be seen, for example, in the concentrations of Spanish-speaking populations in North America or of certain tribal populations in Africa. In such instances, when the Feast is decentralized, the question may well arise as to whether the programme can be conducted in the language spoken by the minority population most prevalent in a neighbourhood. At this stage, the House of Justice does not wish to lay down any hard and fast rules, and it is left to the discretion of the Local Spiritual Assembly concerned to decide, under the guidance of the National Spiritual Assembly, how to address the matter, approaching it with both flexibility and an attitude of learning.
(Universal House of Justice, to all National Spiritual Assemblies, 17 May 2009)

Further, suitable ways should be found to inform believers not fluent in the language in which the Feast is conducted of the content of major messages and announcements. During consultations, they should be afforded an opportunity to express their views, in their own language if necessary. It should be feasible to offer any translation needed in a manner that does not interfere with the smooth running of the meeting.
(Universal House of Justice, to all National Spiritual Assemblies, 17 May 2009)

Naturally, whether the Feast is held centrally or in several locations, a Local Assembly will want all the friends to feel that they are part of one unified community, irrespective of linguistic differences, and will take steps to ensure that an inviting atmosphere is created. To this end, selections from the Writings in the diverse languages spoken by the friends might well be included in the devotional programme of the Feast.
(Universal House of Justice, to all National Spiritual Assemblies, 17 May 2009)

The International Teaching Center has sent us a copy of your letter of 10 October, 1982, asking about language problems brought about by the influx of Iranians who do not understand English. It is important that the Iranian friends be encouraged to make the effort to learn the language used in the country and become integrated into the life and activities of the community. The Nineteen Day Feasts and other official gatherings of the friends should be conducted in whatever is the conventional local language. This does not mean, of course, that at such gatherings some of the readings could not be in the language of the immigrants, or that, if these friends so wish, some classes and conferences may not be held and conducted in their own language for their benefit. The essential thing is, as stated above, to promote the integration of the immigrants into the community and avoid feelings of estrangement or disunity on account of language.
(Universal House of Justice, NSA USA - Developing Distinctive Bahá’í Communities)