MO files: MO, or Machine Object is a binary data file that contains object data referenced by a program. It is typically used to translate program code, and may be loaded or imported into the GNU gettext program.

PO files: PO files are the files which contain the actual translations. Each language will have its own PO file, for example, for French there would be a fr.po file, for german there would be a de.po, for American English there might be en-US.po.

POT file: POT files are the template files for PO files. They will have all the translation strings left empty. A POT file is essentially an empty PO file without the translations, with just the original strings.

• First Step, is to check your site language and for that Go to wp-admin > Settings > General Settings > Site Language
• After Activating Loco Translate Plugin, Click on Loco Translate Plugin and Choose the plugin/theme which provides the text that you want to translate.
• Click on that plugin/theme. Some settings will appear.
• Now, this is an important step. Your site Language decides in which language you have to add translations. For Example: your site language is English(US) and you want to translate text into the German Language. Then Choose the English Language here and add strings translation in the German Language.

• In any case, you will not find your site Language here, then you have to click on New Language and Select your language. This will create a new po and mo file in your language.

• In the above step, I mentioned Global Language Folder and Local language Folder so what it is and which Location we have to choose while creating a PO and MO file of our own Language
• If you want to add your own po/mo files or your custom translation then it is strongly advised that you put set the location as GLOBAL.

For Themes : “wp-content\themes\wplms\languages”

For plugins : “wp-content\plugins\pluginname\languages”

For Themes : “wp-content\languages\themes”

For plugins : “wp-content\languages\plugins”

• The Global translation files are not changed when you update any plugin or theme.
• Only Local Translation files are changed.

• Now, you have your Language PO File, Click on Edit. Now a whole new set of settings will appear.
• Click on the Sync button. This will sync all the strings of the newly created PO File with your POT file. Search for the text that you want to translate.
• select the text. Write the translation text then click on save.
• If you are translating any string with Variables like %s, %d

  Then You need to follow this guide.

  ​https://developer.wordpress.org/themes/functionality/internationalization/#variables​

• Now check the Frontend, you will see your translated text.

• Now start Translating and your translations copied to Global language Folder. No need to translate again.