You can, but it is not straightforward as you may think.

If you press the edit button, on the left hand side of the code, at Line 500 you may find something like this:

return `>>> FULL TEXT of ${letterLabel}: ${messagesText}\n>>> SUMMARY of ${letterLabel}: ${summary}`;

From there forward you see a handful of instructions in plain English that tell the model to generate a summary on the vein of: "Your task is to generate some text and then a 'SUMMARY' of that text, and then do that a few more times..." and so on.

Since this instruction is passed in English, the output will be in English as well. If you want to maintain everything in German, you must translate this instruction to German manually.

Now, you'd be surprised but the summaries may not be the culprit of your run being in English randomly, as this principle applies to the how normal instructions are passed, for example, in Line 7291 of the right hand side of the code, you'll find this:

if(generalWritingInstructions === "@roleplay1") {

And below several instructions in plain English that tell the model how to direct the story. This and several other instructions are passed always each time you press "Send" or the return key, so if you want to be completely sure that your text is never in English, you may need to translate all these instructions as well.

However, something that in the past worked (but I personally have not tested after so many updates this model had undergone so I can't assure it still works) is that in the Custom Roleplay Style box you can in English write as a prime instruction "The whole text, story, RP MUST be in German (or your desired language)" and it would work without need of translating all.

Granted, this will not change the language of the summaries as the instruction for this is done separately, but it may not affect the output that matters for you.

Hope that helps.