How to import translated strings having dynamic variables?
There are two way to achieve this

Add string in a your .po file like follows

msgid "Hi @firstname, please follow @sitelink or mail me at @email"
msgstr "Hola @firstname, por favor, siga @sitelink o envía un email al @email"


and import that .po file by going 


  Administration » Configuration » Regional and language » Translate interface » Import



 If you want to change individual translated strings go to 


  Home » Administration » Configuration » Regional and language »
  Translate interface » Translate


You can find list of strings there each having edit link beside. So you change the individuals like follows.



Hope this helps someone.