"Programmers should speak the language of domain experts to avoid miscommunication, delays, and errors. To avoid mental translation between domain language and code, design your software to use the language of the domain. Reflect in code how users of the softwarethink and speak about their work. One powerful approach is to create a domain model.
The ubiquitous language is a living language. Domain experts influence design and code and the issues discovered while coding influence domain experts. When you discover discrepancies, it's an opportunity for conversation and joint discovery. Then update your design to reflect the results." (Ubiquitous Language in 99 words - James Shore)
End Users are Ubiquitous
Not every software project has a dedicated business analyst or a domain expert. But every software must have users. Also, every software should be written for the users.
In general, on a development team the technical language is understood only by developers, the domain language normally understood only by domain experts. But, to achieve success, the end user language should be understood by everyone. The end user language should be the ubiquitous one.
User Language good in Code?
The whole idea behind an ubiquitous language is that it is pervasive everywhere; in all project related communication, all the way down to the technical level. The idea is to preserve the business intention in the code and have the code mirror the real world as much as possible.
Source code is a little more limited in that you have to avoid keywords on a programming-language-specific basis, but other than that, the advantages are numerous; the programmers don’t have to make the mental translation from the language of code to the language of the domain resulting in less bugs, the domain experts don’t have to read the “Gang of Four” and can more accurately describe their needs, and both programmers and domain experts should be able to understand the end user's language. Driving the design based on the domain in this manner shows how closing the gap between domain and programming language leads to better code.
Most Agilists agree that a shared (ubiquitous) language improves communication between technical and non-technical people. Ensuring a strong tie to the code reinforces the language and ensures it's thorough use.