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Best way to translate or modify theme or plugin strings

All hard coded texts within the theme or plugins can be translated by editing the language po file using one of these tools:

Po/Mo Editor (Free)

Loco Translate Plugin (Free)

PoEdit PRO (Paid) Most recommended. 

WPML Plugin (Commercial)


Screenshots

Loco Translate Example:
http://d.pr/i/149e0
http://d.pr/i/1b9uK
http://d.pr/i/13qA8

PoEdit Example
http://d.pr/i/17Owd
http://d.pr/i/1hNv8