What are good reasons to write my thesis in English?

Clearly, in the time you have performing research and interacting with other people in your field, you were using at least one language, likely more than one given your research on a country that (likely) does not speak either English or German. However, your thesis has some specific audiences to address. If written in German, it certainly addresses your local university department, and the academic community in Germany. Writing it in English will dramatically broaden the potential audience, since many more people in the world can read English as compared with German. But, do those additional people count much? That is up to you to determine, and the university to agree to.

Look at your references, fellow academics you've met at conferences, and those you wish to interact with. Write in the language most likely to be accessible to them.

I am answering your main question (what should I put on the form?), rather than the question of your title (why should I use English?).

The fact that you have to seek permission to submit in English suggests that this is not the done thing in your institution, although it is possible. You should read the form and academic regulations carefully, as these should explain when it is permissible to use a foreign language; if these do not make it clear, ask the relevant academic or administrative staff for clarification. Tailoring your reason on the form to meet their list of acceptable circumstances will increase your chances of being permitted to use English. If do not fall into one of the permitted reasons, you may not have any chance of submitting in English, unless the regulations provide some leeway for discretion.

With the information you have provided, it is impossible to know what the permitted circumstances might be, but here is some speculation:

• If your registered language of instruction is not German
• If appropriate German-speaking examiners will be difficult to find
• If your supervisor is not a German speaker, and cannot adequately support/supervise a German-language thesis
• If you are studying another language, and it is appropriate to write your thesis, or a part thereof, in that language/if you are completing a PhD by translation
• If you are completing a PhD by publication, and your publications are not in German

Given that you state the space available for the justification is small, it's clear that they're expecting a form reason, such as above, rather than a discussion of the peculiar circumstances of your research. As far as I can tell, you've registered at a German institution, where the default language of instruction is German. Unless you were given undertakings to the contrary when you matriculated, I'd say you should be prepared to write a German-language thesis.

In many cases, such a form will be a mere pro-forma requirement. If this is the case at your institution, filling in any of the reasons you mention in the question will do. You wouldn't need to convince anyone, you merely need to provide what is necessary for the box-ticker to tick the box.

However, it could also be the case that there is some real reluctance to let people write in English, and that you actually need to convince whatever decision maker there is. What is a good reason then depends on whatever views these decision makers have. If they are reluctant to allow writing in English, they definitely have weird views, so their notion of good argument might differ from mine.

To figure out whether you fall into Case 1 or into Case 2, and if you are in Case 2, what the succesfull arguments are, speak to your supervisor and to students currently writing their thesis in English.