I have to agree with you.
Collins Dictionary defines "subsection":
a section of a section; subdivision
It is certainly grammatically correct to use either word, and I think it is semantically correct both ways as well. However, as you say, writing "Subsection 2.3" it introducing redundancy, as it is (as you say) blatantly obvious that section 2.3 is a subsection of section 2.
I would say, however, that "Section 2.3" does not imply a subsection, it seems rather explicit that it is a subsection, to me.
A quick search on Google for "Section 2.3" and "Subsection 2.3": Section has 1.2 million hits. Subsection has 46,000 hits. So it definitely makes much more sense to drop the "sub-".
If we have the document:
- Birds
1.1. ParrotsThis is a bunch of information about parrots.
1.2. HawksThis is a bunch of information about hawks.
Both "Parrots" and "Hawks" are sections in their own right. They are simply sections within sections.
This is similar to the folder metaphor in computing. A folder contains many subfolders, but each subfolder is still a complete, and real, folder in its own right.
Princeton University's WordNet defines subsection:
(n) subsection, subdivision (a section of a section; a part of a part; i.e., a part of something already divided)
Furthermore, Merriam-Webster defines section:
a distinct part or portion of something written (as a chapter, law, or newspaper)
So if one takes a section, then takes another logical portion of that, that is another section–also a subsection–that happens to be inside the original section.
E.g. a chapter is a section of a book. A paragraph is a section of a chapter. A sentence is a section of a paragraph. We can logically divide anything into however we like.
Cambridge Advanced Learner's Dictionary defines subsection:
one of the smaller parts into which the main parts of a document or organization are divided
Further details can be found in section 7 subsection 4 of the report.
From the example, we can see that they have referred to section 7.4 as section 7, subsection 4.
My understanding of all this, is that we can call top-level sections, just "section". But we can call non-top-level sections either "section" OR "subsection".
Best Answer
You could use "conclusion" if it's actually a conclusion. A last subsection may well not really be a conclusion.
There's nothing wrong with "section summary" if that's what you mean, but if you do this in every section (or most sections) you could just use "summary". Rather than the "interim conclusion" suggested by your direct translation, "interim summary" would be a more appropriate phrase based on your definition. I wouldn't tend to use this in the case of numbered sections, but you could.
"Recapitulation" is valid but maybe be overly formal or old-fashioned while the more common shortened form "recap" is probably too informal for serious writing.
One option that might work is "summary of section title", e.g.:
Methods to reach the end
1.1. Walk straight there
1.2. Wander about a bit
1.3. Fly
1.4. Summary of end-reaching methods
This has the advantages of reminding the reader what you're summarising, and not filling your contents with a lot of identically-named subsections, whihc doesn't look nice and hinders navigation.