Timing of treatment would be based on several factors. These include the age of the child, when they are starting school, time since onset, reaction to the stutter, severity and how stuttering is impacting on their daily interactions, the severity of the language difficulty and how it is affecting the child’s interactions and whether there is enough language to be able to do stuttering therapy effectively.
If the language difficulties are mild and the child is close to school age then I would treat the stutter first. If the language difficulties are severe and the child is quite young with recent onset of stuttering, then I would probably treat the language difficulty first. If the difficulties were somewhere in between, then I think it would be a case-by-case decision with parental input.
I do think that language development can impact on the severity of the stutter. I have seen children who have a sudden increase in vocabulary or length and complexity of utterances have corresponding increase in the severity of the stutter. I assume that the improvement in language skills has been a trigger for an increase in the severity of the stutter and would manage this with an increase in the structure of the therapy as needed in order for therapy to be effective.