S-2 member registering page

I have a website which is for non-English users. I use S-2 member plugin for them to register an account. I want the registering page shows the new language instead of English.

I was able to find majority of English words on the registering page in the files of the S-2 member plugin and have replaced those words with the new language. However, I am unable to find the two items “user name” and “Email address” in the S-2 member files. -I don’t want to use po/mo file since I only want to spend time to translate the words that the users will frequently see. I guess these two English words may be in wordpress core files, but with so many files, I don’t know where I should look for them. Can anyone give me some instruction? Thanks.

Also for the S-2 member plugin, it only allows the user name to be English letter and lower case. I want it also allow other character. Can anyone instruct me where and how I can change this setting? Thanks.