You have to use quotes. That's how the substitution is formatted.

Valid:
'KEY' => 'This is my language string'

Invalid:
'KEY' => 'This's my language string'

The above is invalid because the string is surrounded by single quote, but there's an unescaped single quote in it which causes it to break out early. It would be escaped as follows.

'KEY' => 'This\'s my language string'

This is just basics of PHP though and not specific to our usage. I highly recommend using an IDE and becoming more familiar with PHP. See the below for some PHP tutorials.

www.w3schools.com/php/default.asp

There's nothing more I can suggest without seeing your full language string. In your case I'm guessing you're hitting something like the following.

Invalid:
"KEY" => "My string [cb:userdata field="cb_fileupload_1" /]"

Valid:
"KEY" => "My string [cb:userdata field=\"cb_fileupload_1\" /]"

The first raw substitution is working fine, but its HTML looks escaped. The second one isn't working because the substitution isn't closed. It needs to be as follows.

[cb:userdata field="cb_fileupload_1" /]

You have the following.

[cb:userdata field="cb_fileupload_1"]

The below may work too.

[cb:userfield field="cb_fileupload_1" output="csv" /]