You could at least justify that the scientific basis for merging is given by the published papers on this topic area. Here are a few examples: https://arxiv.org/abs/2306.01708 https://arxiv.org/abs/2203.05482 https://arxiv.org/abs/2204.03044
Nevertheless, it must be admitted that some merges that should achieve good results on paper only produce gibberish in practice or vice versa. So you probably need a bit of luck ;-)
For the German-speaking world, however, I can definitely say that we are not primarily interested in getting better numbers, but in making the English-language models accessible to the German language, at least to some extent, without completely eliminating their cleverness. So the more intelligent the original English model is before it is fine-tuned with German data, the less stupid the model will be in German, and that is our goal as long as there are no German pretrained models.
Try a few different prompts and let us know what worked for you. For shorter translations, however, it should definitely be sufficient if you keep the system prompt and include the instruction for translating in the user prompt.