I assume the copy/paste suggestion was referring to these forums. I use Chrome w the Google Translate add-on that can translate things as I highlight them. I had to turn off the Discourse (forum) "quote reply highlighted text" feature in Preferences.
Some keyboard apps have a built in clipboard that can be used to copy external text and paste into game chat. This can lead to spam, so be sure to use this power only for good.
I am not aware of any way to select or copy text posted by others in chat. I've had mediocre success stripping text from screencaps using online OCR websites. But language specific ones are hard to come by, nevermind something that can handle a mixture. It's also extremely tedious.
Line does have a Russian translator. Unfortunately I forget exactly how to find them. I know it's an "Official Account" but forget where the search is.