For Windows 10,
1.Use the Windows key + I keyboard shortcut to open the Settings app.
2.Click Time & language.
3.Click Region & language.
4.Under Languages, click Add a language.
5.Click the language you want to add, and select the specific variation if applicable.
6.Select the new language and click Options.
7.On Region & language, select the new language, and click Set as default.
Alternatively, you can select your location to enable the Windows 10 to give you local content.
Step1: Use the Windows key + X keyboard shortcut to open the Power User menu and select Control Panel.
Step 2: Click Add a language, then click the Advanced settings link in the left pane.
Step 3: Click the "Apply language settings to the Welcome screen, system accounts, and new user accounts."
Step 4: On the Administrative tab, under the "Welcome screen and new user accounts", click the Copy settings button.
Step 5: Check the Welcome screen and system accounts and the New user accounts options.
For Windows 7:
Click Start , click All Programs, and then click Windows Update.
In the left panel, click Check for updates, and then wait while Windows looks for the latest updates for your computer.
Windows 7 Language Packs are listed as optional updates. Click optional updates are available.
Under Windows 7 Language Packs, place a check next to the language pack that you want, and click OK.
Last, Click Install updates.
For Windows 8:
Move the mouse pointer to the lower left corner of the screen, right-click, and select Control Panel from the menu.
Under Clock, Language, and Region, click Add a language.
In the Language window, click the Add a language button.
On the Add languages window, scroll down to find the language you want.
Select the language: If the language has no regional variants, click Add.
If the language has multiple regional variants, click Open, select the regional variant you want, and then click Add.
Click Options next to the language you are adding.
If a language pack is available, click Download and install language pack.
If prompted, sign in as an administrator.
When prompted, click Yes to allow Microsoft Windows Language Pack Installer to make changes to the computer.
The language pack begins to install. This might take several minutes.