Learn English – what are the things you put in a showcase at home

All of them sound natural to me except #6. I would say instead:

Why is my stomach burning?

or

What is making my stomach burn?

One other note, though; even though Nos. 1-4 are worded okay, I would not call those "stomach problems." The phrase stomach problems is usually used to describe ailments like indigestion, as you did in Nos. 5 - 8. A growling tummy isn't really a stomach "problem," not in the way I usually hear stomach problems.

Is there any other word which is commonly used to describe stomach related problems?

There probably are, although I think you nailed the two most common ones with Nos. 7 & 8. Upset is often used when referring to the stomach organ, while bloated is sometimes used when referring to the lower abdomen in general, and may not be related to digestive problems at all. Sometimes you might hear the term GI used, particularly in the medical community (GI being short for gastrointenstinal). You can simply Google GI problems for a boatload of examples.

Yes, you're right that a direct translation of "decision guidance documents" is not the best choice. It is certainly to-the-point, but since it's not an established or familiar phrase, its meaning isn't immediately obvious.

Unfortunately this is one of the many times that the best translation must do some rewording, rather than be very word-for-word. To the best of my knowledge there is no direct English corollary for "documents used as the basis for a decision" that fits all contexts generically. There are specialized terms for various contexts, though. For many of the instances you cite, "evidence" or "body of evidence" might be appropriate. "Supporting documents" can be appropriate when referring to less-important documents that accompany a more-important one ("please submit your resume and any supporting documents"). If the emphasis is on a collection of documents that have been brought together into a whole, "file" or "portfolio" are common. Here are some choices for specific scenarios:

• Interviewing a candidate for a job, to refer to the resume/cv/supporting documents: "Looking over your qualifications, I see..."
• Deciding to close a branch of a company: "In making this decision, please consider the file collected over the past year..." (This is another where "body of evidence" might make sense)