I guess it will be not easy to reply to your question.
In my experience, UX approaches and/or methodologies (ex. mobile first) are applied according a mix of inputs coming from several departments, especially in an enterprise environment. As UXers we should take care about all the needs of our stakeholders and define a specific approach and/or methodology according some precise assets (capacity in terms of people for instance).
Anyway, Ive some questions for you:
What do you mean with “internal applications”?
What these apps are supposed to do?
Are they new apps or is an enhancement of existing ones?
What do you , exactly, mean with “enterprise company”? How many users are expected to use these apps?