The Welsh language has been spoken legally for the first time in the European Parliament.
Jill Evans MEP is leading Plaid's campaign for Welsh to be accepted
Plaid Cymru MEP Jill Evans, who leads the campaign for Welsh to be officially recognised by the European Union, spoke in a debate on Wednesday.
Labour's Glenys Kinnock and Eluned Morgan are believed to have been the only two other MEPs who would have been able to understand the speech.
Ms Evans was allowed to speak Welsh under new rules adopted last month.
MEPs are now allowed to speak in a non-official EU language, although their comments are not translated.
Addressing the Strasbourg Parliament's debate on last week's EU summit, Ms Evans said it "needed to see action to improve the relationship and communication between the EU and the peoples of Europe".
"There is no more effective way of doing that than by using peoples own languages.
"That is why I will take the opportunity presented by the decision of 13 October to permit the use of other languages in this house and I will conclude my speech in Welsh.
In Welsh, she then said: "I appreciate the opportunity to be allowed to speak Welsh in this chamber, legally, for the first time, even if there is no translation.
"This is a step forward towards a Europe of the peoples and a Europe that celebrates its diversity. Culture is the fourth dimension in building Europe - in addition to the economic, environmental and social dimensions.
"The recent EU summit put considerable emphasis on communication. What better way is there than to communicate with people in their own languages."
Responding, Dutch Prime Minister Jan Peter Balkenende said that he would take note of calls for improving the status of Welsh and Irish in the EU's communications.
Plaid has said that, if Welsh was adopted as a formal EU language, it would mean that Welsh speakers could write to European institutions and expect a reply in their language.