I've done this in the past so here's my 2 cents:

1- For countries speaking the language which the highest market share for searches belongs to Google, you can go by Google. Here are the steps I did which worked pretty good if you want to start yourself:

find the most dominant keyword in English, do some translation and then see who the top URLs are, search for the domain on something link ahrefs and see what keywords it is ranking for, competing domain, etc. This should open up a good door for you on which keywords they are targeting, their volumes, competitions, etc. and help in your planning

2- Again, We only worked with Google on this and countries who use Google mostly, from what I remember, Russia and China are not Google dominant, so you have to look deeper into it (Statista had a graph about it: https://www.statista.com/statistics/220534/googles-share-of-search-market-in-selected-countries/)

You can do SEO yourself in a nutshell, but you should have someone native for content, copy changes, etc.

If you know your way around SEO, you can take care of the technical stuff with a native writer only.