Welsh and other Celtic languages are hardly obscure languages:
*These languages have long been written. Official dictionaries around the languages have also likely been around for centuries, probably all over the world.
*There are plenty of people of Celtic descent in many ex-colonies outside Europe. Some even still speak Celtic languages, as in Patagonia, and Gaelic speaking parts of Nova Scotia, Canada.
*There are also a lot of simularities between these languages and a great deal of other Western languages. For example, the Welsh word for 'cat' is cath. It sounds like the English word but with a th-sound where the English word has a t-sound. Incidentally, the Navajo word for cat, moasi, was one of the Navajo code words for the letter Charlie. Use the Welsh word for 'cat' like that and its meaning would be all to obvious.
What makes a language obscure, apart from not being widely spoken or understood outside the homeland of its speakers, is a lack of a common basis of familiarity with any more widely known language, like English, German, Russian, French, Spanish, Japanese, or even any Celtic language.
The nearest thing to a truly obscure language anywhere in Europe is the Basque language. But even this language has been written for a long time and there are Basque speaking minorities scarrered around ex-colonies of European countries. I'm not sure if any truley obscure language even exists in Asia or North Africa either. All are either widely known well beyond the home territory of native speakers or are not so well known but with a lot of simularities with such widely known languages.