Although your Mac is set to display the language of the country or region in which it was purchased, you can choose a different language to use. For example, if you bought your Mac in the United States but you work primarily in French, you can set your Mac to use French.
You can also choose different languages for individual apps. For example, if your system language is set to Simplified Chinese, but you prefer to use a certain app in English, you can do so.
Change the system language
On your Mac, choose Apple menu 
> System Preferences, then click Language & Region. Do any of the following: Add a language: Click the Add button 
, select one or more languages in the list, then click Add.
The list is divided by a separator line. Languages above the line are system languages that are fully supported by macOS and are shown in menus, messages, websites and more. Languages below the line aren’t fully supported by macOS, but may be supported by apps that you use, and shown in their menus and messages and on some websites. If you can’t use the input source that’s selected in the Input menu to type a selected language, a list of available sources is shown. If you don’t add an input source now, you can add it later in the Input Sources pane of Keyboard preferences. Change the primary language: Drag a different preferred language to the top of the Preferred languages list. If macOS or an app supports the primary language, menus and messages are shown in that language. If it doesn’t, it uses the second language in the list, and so on. The language may also be used on websites that support the language. The order of the languages in the list determines how text displays when you type characters in a script that belongs to more than one language. See If international fonts don’t display correctly. If your Mac has multiple users and you want everyone to see the language you chose as the primary language in the login window, click the Action pop-up menu , then choose Apply to Login Window.
Choose the language you use for individual apps
On your Mac, choose Apple menu > System Preferences, then click Language & Region.
Do one of the following: Choose a language for an app: Click the Add button , choose an app and a language from the pop-up menus, then click Add. Change the language for an app in the list: Select the app, then choose a new language from the pop-up menu. Remove an app from the list: Select the app, then click the Remove button 
. The app uses the default language again. If the app is open, you may need to close and then reopen it to see the change. Источник
Change the language used at the login screen on your Mac
The login screen is displayed in the language you selected the first time you set up your Mac. You can change it using these steps.
If there’s only one user account on your Mac
- Choose System Preferences from the Apple menu.
- Click Language & Region.
- Drag the desired language to the top of the Preferred languages list. If the language you want isn’t listed, click the Add (+) button to add the language you want.
- Restart your Mac.
After your Mac restarts, the login screen uses the language you set.
If there’s more than one user account on your Mac
If there’s more than one user account on your Mac, or you want to change only the language at the login screen, use these steps:
- Log in as an admin user.
- Open Terminal from the Utilities folder.
- Type or paste this command in Terminal:
- Press Return.
- When prompted for a password, enter an administrator account password and press Return.
- A list of available languages is displayed. Type the number that appears next to the language you want to use, then press Return.
- Quit Terminal.
The next time you log out or restart your Mac, the login screen uses the language you set.
Change your keyboard layout
To select a different input method or keyboard at the login screen, click the Input menu in the upper-right corner of the menu bar and choose an option.
If you don’t see the Input menu, use these steps to add it to the login screen:
- Log in.
- From the Apple menu, choose System Preferences.
- Click the Users & Groups icon.
- Click the lock icon. Enter an admin user name and password.
- Click Login Options.
- Select the option to Show Input menu in login window.
How to Change Mac System Language via Command Line
Polyglots, learners, and the bilingual often want to change their Mac system language for obvious reasons, but another less obvious situation can occur if you’re troubleshooting a Mac that either originates in another country, had the language changed at some point, or is just set to a different language.
Since it can be pretty challenging to navigate around in a language you don’t know at all or barely understand, it may be difficult to add and change the language in Mac OS X through System Preferences, and sometimes that easy approach is just downright impossible or inaccessible, perhaps because you’re reinstalling system software, or booted into Single User Mode for troubleshooting purposes.
But not to panic, you can also change the system language used on a Mac from the command line, handled entirely from the Terminal. This means you can either change the language directly from an active Terminal app, or even through ssh remotely if need be.
Changing the System Language in Mac OS X via Command Line languagesetup Tools
This works in all versions of Mac OS X, whether it’s a version called MacOS, Mac OS X, OS X, or BoggleTurkeyOS.
- Launch Terminal if you have not done so already (or ssh if you’re doing this remotely)
- Type the following command then hit return:
* The complete list of number codes plus the associate language is listed below, this should make it easier to navigate if you find yourself on a Mac set to a non-native language.
1) Use English for the main language
2) Utiliser le français comme langue principale
3) Deutsch als Standardsprache verwenden
4) 以简体中文作为主要语言
5) 以繁體中文作為主要語言
6) 主に日本語を使用する
7) Usar español como idioma principal
8) Usa l’italiano come lingua principale
9) Gebruik Nederlands als hoofdtaal
10) 주 언어로 한국어 사용
11) Usar português do Brasil como idioma principal
12) Usar o português europeu como idioma principal
13) Brug dansk som hovedsprog
14) Käytä pääkielenä suomea
15) Bruk norsk som hovedspråk
16) Använd svenska som huvudspråk
17) Сделать русский язык основным языком системы
18) Użyj polskiego jako języka głównego
19) Ana dil olarak Türkçe’yi kullan
20) استخدام اللغة العربية كلغة رئيسية
21) เลือกภาษาไทยเป็นภาษาหลัก
22) Vybrat češtinu jako hlavní jazyk
23) Magyar kiválasztása alapértelmezett nyelvként
24) Seleccioneu el català com a idioma principal
25) Odaberite hrvatski kao glavni jezik
26) Επιλέξτε Ελληνικά ως την κύρια γλώσσα
27) בחר/י עברית כשפה ראשית
28) Selectați româna ca limbă principală
29) Vybrať slovenčinu ako hlavný jazyk
30) Вибрати українську основною мовою
31) Gunakan Bahasa Indonesia sebagai bahasa utama
32) Gunakan Bahasa Melayu untuk bahasa utama
33) Sử dụng Tiếng Việt làm ngôn ngữ chính
34) Utilizar español de México como el idioma principal
q) Quit
This is obviously more technical than adding a new language or changing the MacOS system language it from the preferences panel in Mac OS X, but it can be extremely helpful for some unique troubleshooting situations. Remember you can also switch the keyboard language quickly with a keystroke if you frequently switch languages that are being typed as well.