Looking for summer internships

Firstly, welcome to WSO. Now to your queries,

• If you lack any prior internship experience, it is extremely hard for you to secure a decent S&T internship at these trading desks (e.g. Cargill, Glencore) unless you have some serious connections. You'll have better luck searching for a internship at a boutique trading desk or a PWM gig.
• It's good to see that you're reading up about these firms and industries that you're interested in. This will come in handy when you ACTUALLY get interviews.
• The only way for you to get any decent S&T internships is to network, network and network. It's a little late now especially it takes time to build connections, so you might want to start now for junior year summer internships, or ask your family members/relatives for contacts at trading desks.
• Every single person who is applying for a trading internship will tell you that they are 'highly motivated and informed and could be a great asset to them'. Talk is cheap, you have to PROVE IT.
• Your major in Russian language might be of use for Investment Banking gigs (MAYBE), but for S&T, all you need to know is the English language and math. (side note: if you're thinking about leveraging your Russian language skills to get a IB gig, make sure it is of working proficiency i.e. read, write and speak. You might even have to translate a document written in Russian to English)
• Using LinkedIn is a good idea for networking. I have personally used it to network with a couple of recruiters and there's nothing wrong with the idea. Think about it, you lose little in terms of time spent on contacting these people, but have so much to gain if it pans out for you, the risk/reward is skewed in your favor. Go for it. Start by asking alumni etc for informational interviews and prepare well for them.

Personally, I'd recommend that you visit the older threads on this site for more understanding about the recruiting process/background needed for S&T internships. There's a wealth of knowledge lying on this site, so exploit it to the fullest. Just my two cents. All the best!