I think your are focusing on a wrong point with your question. Instead of thinking on how to get more pictures at SS, it would be more useful and efficient to work on "how can I produce better photos". Just MHO.

I don't think I was understood correctly. Perhaps my statement was too generic

I did not mean at all to start discussion about creative photography and whether is suits microstock.

What I meant is that focusing on SS acceptance criteria alone, in isolation from other aspects is not efficient. First, it will not help you to get very many sales. Second, it will not help you to get your images accepted elsewhere. Getting most your photos accepted just to SS is actually a pretty simple and rather technical task.

What I am saying is that getting good quality photos that will be accepted at most microstocks is more efficient approach.  I am talking about "good quality photos" meaning "photos suitable for microstock". If you understood "good quality photos" as blurry/grungy/out of focus "creative" pictures - this is your problem, I didn't say that and I didn't mean that. If you understood that as "sterile" images made with F32 (which is technically challenging with most lens being not so sharp when completely closed) this is also your problem - I didn't mean that.

What I mean is that a "good quality photo" is well exposed (no clipped lights/blacks and correct gamma), is sharp, not noisy (it does not need at all to have a "plastic" look though). It should not look like a family/touristic snapshot; and the lighting of the scene should be good.

As long as the picture doesn't make an impression of an amateur snapshot, and fits the abovelisted technical criteria, it is what I call a "good quality photo". The content can be quite different. It can be creative - contrary to stereotypic opinion stocks do accept creativity and do sell it. They don't always accept all types of creativity, but they do accept much.

I think your are focusing on a wrong point with your question. Instead of thinking on how to get more pictures at SS, it would be more useful and efficient to work on "how can I produce better photos". Just MHO.

do you know if keywords entered in English are somehow translated to German for German buyers?

I did some tests myself, already a year or two ago. I compared Genuine Fractals, Bicubic Resizer, and also some other tools (don't remember their names).

My conclusions are:
a) there is no notable difference if upsizing is not too big
b) when upsizing too much, the quality is not acceptable anyway, regardless the software used.