Programming and reading.
Latvia is partly involved, yes, but it’s also part Russian and recently moved to Singapore. You may find the history section on Wikipedia interesting; it also lists the russian part-ownership as reason for many users leaving OnlyOffice (and I’ve seen quite a few posts on that at the time).
As for the open-source part, I stand corrected, thank you
From what I can tell, OnlyOffice has the best compatibility and the nicest UI (similar to MS office), same as with the regular applications. NextCloud Office is based on LibreOffice (officially Collabora, which is their name for the web product), so again same as the regular applications you’ll have some compatibility issues. That said, if you don’t need compatibility with existing documents or only documents made with LibreOffice, either is fine.
One concern many have is that OnlyOffice is closed source (edit: my bad, it’s been open-source for a long time) and russian based (edit: partially russian, see Wikipedia), while LibreOffice is open source.
Did you really mirror the image part to repost this? Even the watermark is flipped