1. Download the latest translations from
https://crowdin.com/download/project/electrum.zip (If you would like to contribute your translation to the project, sign up for free with crowdin to translate directly to the project.
https://crowdin.com/project/electrum )
2. Open any of the current languages files from the electrum-client folder in poeditor (Every po file contains the English strings in it) and change the language setting to the language you want to translate into, then edit each string.
3. In ./lib/i18n.py (you will need to run Electrum from source) you must modify the languages dict at the end to include your language. Then you must include your language's folder and electrum.po file in the ./locale directory.
Then you should be able to run Electrum in your language.
But like I said before, it would be much easier for you to contribute by just translating your language on Crowdin. We would greatly appreciate it.