WotLK How can I change my in-game Language of UI to Russian or any other Language?

There are different language clients, but enUS client is most stable so that is what you want to use, with LAA the enUS client never crash while some of the other ones do. So what you want to do, is use enUS client, but just change it to another language.

EDIT: Further research show that deDE/esES/esMX/enGB/enUS/frFR/ruRU clients are all supposed to have identical executable when they're clean, so the problems were with unclean clients I suppose. zhCN/zhTW client executables I don't know whether they're different or not.
Therefore I updated the guide below, he can just download ruRU client and use it.

Here's where to download ruRU language client:

(3.3.5a ruRU client) (checksums)

Here's how to change language of existing client to something else (not needed if you download ruRU client from the start):

1. Copy the DataruRU folder from the Russian client over to the English one.
2. Edit WTFConfig.wtf and change SET locale "enUS" to SET locale "ruRU"
3. Done. You can now delete the DataenUS folder if you wish to remove the English language.

The same instructions work for other languages too, just replace ruRU with deDE, frFR, esES, zhCN, etc.

You also want to patch the WoW.exe with the "Large Address Aware" aka "4gb patch", so it can use 4GB memory and doesn't crash in dalaran: https://ntcore.com/4gb-patch/