Tagging - a never ending story

So let's first identify the aspects that tend to cause incongruity - not alone from user to user but often within one single person's documents.

1. Singular or plural?
2. One or several languages?
3. Descriptive or interpretive?
4. Nouns or adjectives?

Delicate Dandelion

(1) Say there is a flower on your picture, you will probably apply the tag "flower". The same if there is a dog or a house or a table - you name it. But what do you do when there are two of the same on the picture? Do you then use "dogs" or do you stick with "dog"?
It may seem unimportant but this is probably one of the main sources of inconsistencies - and it is hard to give one definite answer as to how it should be treated.
I tend to favour using singular in all cases. First, the singular describes it correctly even if there are more than one object of the same sort - but then again, plural incorporates a singular object as well. Second, the singular is the radical of the concept/term and thus a common denominator.
But does it always serve the purpose?
Sometimes it might be important to make it clear that there are several dogs on a picture and not just one. You might want to find a picture of a pack of dogs and not of a lonely stray dog.
Then again, searching might be "insensitive" and not look for the exact phrase and therefore also present you with results tagged "cats" when you asked for "cat".
You see, I am not really sure about this one and will - as with the other three aspects - be very grateful for comments.

(2) The question of language is not only limited to tags, it also goes for titles and descriptions but I think it is most important in the field of tagging your documents.
Usually I am a big friend of my mother tongue (German) and try to use it as often as possible while avoiding foreign words. Still, in the case of applying a tag to a document I favour the English language.
Of course I could also put the German expression along with the English word - but where should that stop? I also know French and a little Spanish. I could also easily look up the translation for a dozen other languages. Should I stick to the European continent? Or all languages with Latin characters? Left to right? You see, it would probably go on forever and I'd end up with a list of hundred tags for just one object.
Since the "language of the internet" - I know, this can be discussed, too - is English and English is a language many people are acquainted with, I have decided to exclusively use it for tagging documents.
I would also love if everyone else did the same because then I could be sure to really find every picture with a cat on it when searching for the tag "cat" - not having to also look for "katze" or "chat".
Of course this could be interpreted as yet another piece in the puzzle of domination of the English language on the net and as disrespect to other languages - but I am just trying to be pragmatic here.

Autumnal Melancholy

(3) So I have this atmospheric picture of an autumn scenery that I consider to be "melancholic". Do I use this word as a tag for the picture? "Melancholy" is not an object but rather a concept. It cannot be "seen" on the picture. It can only be "felt".
But does that qualify as a tag? It is definetly an interpretation of that which is actually "on" the photo. And it is of course my interpretation. Other people might interpret it differently or use another word to describe their feelings with that particular picture. Feelings can easily be misunderstood, too.
Should you therefore only use descriptive tags - tags that objectively name what is really there? For the sake of not being misunderstood?

(4) This aspect is sort of connected to the question of descriptive vs interpretive insofar as it can confine and restrict the pool of possible tags.
Should only objects displayed in a picture be used as tags or should their properties also be named? Does it make sense to use adjectives such as "large" or "small" or "wet" or "dry"? Adjectives are difficult to use on their own because they need some object they refer to, like the "dry board" or the "small apple". Alone they don't mean much. Would it make more sense to speak of "humidity" instead of "wet"? Then again, that only works if there is humidity as a whole to be seen and not only one or two drops of water on some object. The "Wetness" then becomes an object itself. Or should adjectives be combined with the noun they describe, like the "wet board"?
I tend to exclude adjectives all together and only use nouns. All other options either are meaningless or complicate the pool of tags even more since there are limitless possibilites of combinations of just one noun with descriptive adjectives.

Probably all of the questions I posed are related to yet another question:
Do you tag for yourself or do you tag for other people?

Especially aspects (1) and (2), singular/plural and language, are essential when tags are supposed to help other people find documents from you. They need to know how to search and what to expect. If they can rely on the fact that you are using English words, things get a lot easier. If they know you are consistently using the singular - even though there might be a group of the tagged object - they won't have to to bother trying out both forms.
If you are tagging your documents for your own personal use, to help yourself find things, then it is pretty much up to you and your way of classifying things - much as you are doing at home at your desk. You will probably instinctively use the proper terms to tag and then later search for documents without much trouble - simply because it is your own inner logic at work.
But then again - over time your repository of documents will grow and your habits might change. It will certainly also be of avail to you if you use tags in a consistent way.

So here we are - at the end of one long blog entry.
I hope you made it through without rolling your eyes too much at possible laughable simple and banal questions - but they still confuse me and I would really like to work out some "convention" for using tags so they can unfold their full potential.

I would heartily appreciate any comments and suggestions!