To change admin user language navigate to Edit Profile on top right corner & select your language. The selected language will be set for the logged in admin user.

Default system language can be setup from Settings → Localization Settings
Language files are located in system/i18n/
If you want to improve language file, find your language file in system/18n/ folder & edit this file using any text editor. For example to edit the Netherlands language file, edit nl.php file.
If you would like to override a string from the default translation, you can override a language string.
To do this, start by renaming the file- system/overrides/i18n.sample.php to i18n.php and add the string you want to translate in the return statement.
Available language file for the translation-
| Language Name | iso code |
|---|---|
| العربية | ar |
| বাংলা | bn |
| čeština | cs |
| dansk | da |
| Deutsch | de |
| Ελληνικά | el |
| English (UK) | en_gb |
| English (USA) | en |
| English (AU) | en_au |
| Persian (Farsi) | fa |
| español | es |
| español latinoamericano | es_419 |
| español de España | es_es |
| suomi | fi |
| Filipino | fil |
| français | fr |
| עברית | he |
| हिन्दी | hi |
| magyar | hu |
| Lao | lo |
| Bahasa Indonesia | id |
| italiano | it |
| 日本語 | ja |
| 한국어 | ko |
| Bahasa Melayu | ms |
| norsk bokmål | nb |
| Nederlands | nl |
| Norwegian | no |
| polski | pl |
| português | pt |
| português do Brasil | pt_br |
| română | ro |
| русский | ru |
| svenska | sv |
| ไทย | th |
| Türkçe | tr |
| українська | uk |
| Tiếng Việt | vi |
| 中文 | zh |
| 中文(台湾) | zh_tw |