[jcranmer]

Welsh is generally highlighted as the example of a successful language revitalism movement, but it's also one of the rare examples of such movements succeeding. By contrast, you can look at Irish--where the need for the language that wasn't English was seen as absolutely essential as part of the (successful) revolution and independence movement--and see that the language revitalism there is more or less a failure. A century after independence, the number of L1 speakers of Irish has gone down, and I believe the Irish government still conducts most of its business using English (despite English officially being the lesser of the official languages) since so few members of government are sufficiently proficient in Irish.