The 2000 photos I have on the top tier sites are doing very well, but istock always falls behind a little; if ss would be 100, then istock never is higher than 25% for me.
I am Dutch and use the keyword tool to add keywords to my images, always in english because the 'automatic' translation from dutch to english is a true nightmare.

Could it be that it makes a difference if you choose UK English or US English in Deepmeta; up until a few months ago I uploaded with US English and switched to UK English for now and it looks like - maybe - it is getting a little bit better.

I know that the istock search engine should handle all languages alike, but what would you think is the best option?

It shouldn't make any difference, though as noted above, there is a slight difference. I have set UK (British) (not in DM, I upload directly, but it shouldn't make any difference) and 'thought' that if I put in a US word or spelling that it would show up anyway as English (US) - I'm pretty sure that used to happen, but it certainly doesn't happen now.
As an experiment, I just tried sidewalk and pavement on a file, both showed up as UK (British) and mapped to 'pedestrian walkway' (strange, I'd think a 'pedestrian walkway' was something else completely!), so whether you search on sidewalk or pavement you should find the same file. Also color and colour ditto.

It's not you, it's iStock. I and many others (not quite all) are finding that recent-ish files (over the last three years) aren't selling, especially as credit sales. I'm hardly uploading there now, but of the files I've uploaded this year I have only a tiny trickle of credit sales and >10x the number of subs sales (which is unacceptable and why I've all but stopped uploading). Files uploaded in 2014 aren't much better.

If you find the translation from Dutch to English to be inaccurate, please post the info, with some strong examples of error, on their keywording/search forum, if you haven't done so already.

It shouldn't make any difference, though as noted above, there is a slight difference. I have set UK (British) (not in DM, I upload directly, but it shouldn't make any difference) and 'thought' that if I put in a US word or spelling that it would show up anyway as English (US) - I'm pretty sure that used to happen, but it certainly doesn't happen now.
As an experiment, I just tried sidewalk and pavement on a file, both showed up as UK (British) and mapped to 'pedestrian walkway' (strange, I'd think a 'pedestrian walkway' was something else completely!), so whether you search on sidewalk or pavement you should find the same file. Also color and colour ditto.

It's not you, it's iStock. I and many others (not quite all) are finding that recent-ish files (over the last three years) aren't selling, especially as credit sales. I'm hardly uploading there now, but of the files I've uploaded this year I have only a tiny trickle of credit sales and >10x the number of subs sales (which is unacceptable and why I've all but stopped uploading). Files uploaded in 2014 aren't much better.

If you find the translation from Dutch to English to be inaccurate, please post the info, with some strong examples of error, on their keywording/search forum, if you haven't done so already.

You are absolutely right in the old files selling way better in istock; but I still upload hoping things will turn somewhere in the future.
Difficult still which to chose, uk or us.

And for Dutch: if for instance you visit the ss site in dutch (default when your ip is dutch), the keywording and searching is a total mess, not more than 5/10% is translated correctly with their automated system. A photo of a plain house with normal correct keywords will suddenly have strange dutch keywords like 'unexperienced', 'motherland', 'green boy', etc. (one of my own files, and those keywords don't show up in the english version), so I never do my keywording in Dutch and let some machine do the translation. It would probably ruin my sales..