I am currently doing my master’s in German and need to start writing my thesis. Although I am already comfortable speaking the language, writing (especially academic papers) is not something I feel great about. Also, my thesis is a big deal. Legally speaking, could I write it in English and have it translated? It would still be my work.
This is not a legal issue—at least not at first. If you have copyright of your work, you can at any point in time—even years from now—sign a contract with a professional who will translate the thesis into another language and provide certification of having done so. That would be perfectly legal.
The real question is if your faculty will allow you to have someone else translate your thesis so that you can submit it in German, which is largely an administrative decision that may depend on the university. If it were professionally translated, you would have to disclose that fact as part of the submission since it would technically not be entirely your work. Most theses require such an explicit statement as part of the submission.
There may be an alternative: you should consult the relevant Prüfungsordnungen (examination regulations) that cover your program. Many programs now permit theses to be submitted almost entirely in English along with a short amount of material in German (usually something like an abstract or summary plus any “pro forma” statements. It may not be necessary to hire a translator if you can do the basic translation and submit in English.
I really need someone to create an "ask your advisor" sticker or something...
No, seriously, ask your advisor. He/she will know the rules, he might even be totally ok with an English thesis, it is not uncommon to write in English, especially if you are planning to publish it or part of it later. Furthermore, even if you would be allowed to write it in English and have it translated later by law (which I don't know for certain), you don't know if your translator properly translates all the technical terms and it would be an insult on your supervisor to force him to accept it because he has to by law, without talking with him first.
When I had to do my project in French - I wrote in French directly trying to minimise my errors - as it was engineering based the vocabulary is more difficult...
But as I completed each chapter / section I got my French colleagues (also students) to read and correct / improve / comment on what I was writing.
I bribed them with food and wine :) :) But we also had an exchange where I would look at the the English they had to write and help them with that...
We helped each other...
In my experience (fluent in Spanish, German and English) is that it is much harder to translate than to write in the target language in the first place. There are too many colloquialisms, figures of speech, exact technical phrasings that are next to impossible to translate decently (if at all). And the translator will have to be familiar with both languages (hard to get) and very familiar with the subject (that presumably leaves anybody but yourself and perhaps a handful others straight out).
I'd try first to write in English (fun fact: was strictly forbidden here, but often done anyway as many theses were published as papers elsewhere, or were written as part of international collaborations; local language is Spanish!). You can do your presentation in English or German at the end. Ask your advisor.
Second choice, and like others tell you, write in German, check with available tools (spelling, ...). Ask your colleagues/co-students to read chapters (both for language and contents). Bribe them by throwing a party for helpers if need be, make sure to credit their help.
Only as a very last resort would I reach out to a translation service.
First of all you need to weigh in pros and cons of having the final version of your thesis in German vs in English. Think of your future career plans in Germany or elsewhere, language the defense committee is comfortable with, etc.
If you decide to write it in English and translate into German, do the following:
Make sure the thesis is complete, nice, and shiny in English
Translate it roughly by yourself into German with the help of fine free tools such as Google Translate and text editor spell checker, etc. Read it and see if you can correct some translation by yourself.
Buy a translation service to edit and proofread your translation. You will supply the translators with your German version and well as the English version.
That way everything is perfectly legal, ethical, moral, transparent, and makes total sense on your side. Now, whether the translators will edit your German translation or completely discard it and translate it on their own isn't really of your concern, in fact it is their own business.