Typing in Chinese is actually very easy. There's a system called pinyin used to transcribe Chinese phonetically using Roman letters. You can use any old American keyboard, no problem. Basically I switch to "Chinese mode" on my computer or phone, type a sentence/phrase like "dajiahao" in pinyin, and the computer will guess which characters I am looking for. It also provides other characters with the same pronuciation and if necessary you can scroll through to find the right one.
I don't know about Spanish or russian, but I'm sure there are ways to configure your pre-existing keyboard.
The long (and tiresome) way is to use the Character Map program on Windows... that is, if you use Windows.
You can add extra keyboard layouts and toggle between them. I have about 10 of them set up for different languages. How to do this depends on your system, so here is where you need to do some online searching.
Keyboard stickers are available if you want to make your keyboard "bilingual". You can search for these as well.
To be honest, the easiest way is learn how to touch type; don't worry about the speed. I use a Turkish keyboard but often type in Russian. I simply make myself remember what letter is where.
(For Chinese, look up "bopomofo" as one option.)
I think I might have found my own answer here:
Found what I needed here (and more) https://www.branah.com/russian
This is my own keyboard: https://www.branah.com/swedish
Peachey gave a good explanation. I'll add two more comments:
I've heard of some people printing out the keyboard layout and taping it to their computer screens instead of buying stickers. However, I imagine this would take someone a bit longer to type. Stickers are usually only a few dollars.
You can also use ALT codes. In some cases where the alphabet of your target language is close to your native language, it can be fairly easy to use. I use ALT codes for Spanish rather than adding an alternate keyboard layout.