Language Spreads for Practical Reasons much Faster than Imposed Ones
The number one reason for a language to spread quickly is because someone has a thing that is so important to other people that they WANT to learn their language to be able to negotiate for, or use it. So if you want Celtic to be spoken outside of Lloegyr, it means that the this new nation has something very important that the rest of the world wants a part of.
One solution would be to make Lloegyr the pioneers of some Unobtanium or a McGuffin Tech that forces the rest of the world to trade with them or be left behind.
Another way you could go is to treat Lloegyr as a sort of preserver civilization. Instead of just having Europe collapse, it could have been a global event. The whole world got caught up in some very nasty wars. Here in the info age, the biggest targets of military aggression are the electric grid, communications networks, and data centers. So imagine that nearly all of infrastructure of the internet gets wiped out leaving vast amounts of human information destroyed or otherwise inaccessible. 209
But somewhere in Ireland, perhaps there are data centers used to store all the Gaelic translations of nearly every website in the world. Nearly no one speaks Gaelic; so, these data center were deemed too unimportant to destroy. But when the fighting is all done, it turns out that Ireland not only has the most complete record of remaining human knowledge, but it is in Gaelic. It also turns out all that fancy translation software that companies like Google has been developing for the past 20+ years were also stored and backed up in now destroyed data centers; so, just converting the Gaelic translations of the internet back to other languages is not so easy as it once was. As various factions try to rebuilt the internet, most find they are starting at square 1, or only have bits and pieces of what they need, but the <PERSON> have everything they need to restore most human knowledge. So, with the internet mostly back up, but only in Gaelic, there would be a huge impetus for people across the world to learn Gaelic so that they can use the best version of the restored internet there is. 111