jqGrid for ASP.NET comes with a large number of predefined language packs and settings for almost all popular languages. They are located in the installation package, in the /js/i18n folder. Lnaguage packs are in the form of javascript files, containing definitions for all strings in the grid that can be localized - this includes messages, captions, paging information, search/add/delete dialog labels, etc.

In order to use a particular language pack, you need to include the javascript language pack to the head of your page, after the jQuery library reference (since language packs depend on jQuery) and before referencing the jqGrid javascript file (since it is dependent on the language pack).
<%@ Page Language="C#" AutoEventWireup="true" CodeBehind="default.aspx.cs" Inherits="JQSuiteASPNETExample.examples.functionality.localization._default" %>
<%@ Register Assembly="Trirand.Web" TagPrefix="trirand" Namespace="Trirand.Web.UI.WebControls" %>

<!DOCTYPE html>
<html lang="en-us">
<head id="Head1" runat="server">
    <title>jqGrid for ASP.NET - Localization</title>
    <!-- The jQuery UI theme that will be used by the grid -->
    <link rel="stylesheet" type="text/css" media="screen" href="http://code.jquery.com/ui/1.12.1/themes/redmond/jquery-ui.css" />
    <!-- The jQuery UI theme extension jqGrid needs -->
    <link rel="stylesheet" type="text/css" media="screen" href="/themes/ui.jqgrid.css" />
    <!-- jQuery runtime minified -->
    <script src="/js/jquery-3.2.1.min.js" type="text/javascript"></script>        
    <!-- We will use this literal to inject the respective localization javascript at runtime -->
    <asp:Literal runat="server" ID="localeLiteral" Text="<script type='text/javascript' src='/js/trirand/i18n/grid.locale-en.js'></script>" />    
    <!-- The jqGrid client-side javascript -->    
    <script src="/js/trirand/jquery.jqGrid.min.js" type="text/javascript"></script>
    
    <style type="text/css">    
        body, html { font-size: 80%; }    
    </style>
</head>
<body>
    <form id="form1" runat="server">        
    
    <asp:RadioButtonList runat="server" ID="LocaleRadioList" RepeatDirection="Vertical" RepeatColumns="6" AutoPostBack="true" style="font-size: 100%; font-family: verdana;">
        <asp:ListItem Value="grid.locale-en.js" Selected="True">English</asp:ListItem>
        <asp:ListItem Value="grid.locale-de.js">German</asp:ListItem>
        <asp:ListItem Value="grid.locale-fr.js">French</asp:ListItem>
        <asp:ListItem Value="grid.locale-sp.js">Spanish</asp:ListItem>
        <asp:ListItem Value="grid.locale-it.js">Italian</asp:ListItem>
        <asp:ListItem Value="grid.locale-nl.js">Dutch</asp:ListItem>
        <asp:ListItem Value="grid.locale-no.js">Norwegian</asp:ListItem>        
        <asp:ListItem Value="grid.locale-fi.js">Finish</asp:ListItem>
        <asp:ListItem Value="grid.locale-he.js">Hebrew</asp:ListItem>
        <asp:ListItem Value="grid.locale-jp.js">Japanese</asp:ListItem>
        <asp:ListItem Value="grid.locale-pt.js">Portuguese</asp:ListItem>
        <asp:ListItem Value="grid.locale-pt-br.js">Brazilian-Portuguese</asp:ListItem>
        <asp:ListItem Value="grid.locale-ru.js">Russian</asp:ListItem>        
        <asp:ListItem Value="grid.locale-ro.js">Romanian</asp:ListItem>
        <asp:ListItem Value="grid.locale-pl.js">Polish</asp:ListItem>                
        <asp:ListItem Value="grid.locale-cs.js">Czech</asp:ListItem>        
        <asp:ListItem Value="grid.locale-el.js">Greeek</asp:ListItem>        
        <asp:ListItem Value="grid.locale-fa.js">Persian</asp:ListItem>        
        <asp:ListItem Value="grid.locale-is.js">Icelandic</asp:ListItem>
        <asp:ListItem Value="grid.locale-sv.js">Swedish</asp:ListItem>
        <asp:ListItem Value="grid.locale-tr.js">Turkish</asp:ListItem>
        <asp:ListItem Value="grid.locale-ua.js">Ukranian</asp:ListItem>        
        <asp:ListItem Value="grid.locale-cat.js">Catalan</asp:ListItem>
        <asp:ListItem Value="grid.locale-bg.js">Bulgarian</asp:ListItem>        
    </asp:RadioButtonList>
    
    <trirand:JQGrid runat="server" ID="JQGrid1" DataSourceID="LinqDataSource1" Width="600px">
        <Columns>
            <trirand:JQGridColumn DataField="CustomerID" PrimaryKey="true" Width="130" />
            <trirand:JQGridColumn DataField="CompanyName" />
            <trirand:JQGridColumn DataField="Phone" />
            <trirand:JQGridColumn DataField="PostalCode" />
            <trirand:JQGridColumn DataField="City" />
        </Columns>
        <ToolBarSettings ShowSearchButton="true" ShowAddButton="true" ShowEditButton="true" />
    </trirand:JQGrid> 
    
    <asp:LinqDataSource ID="LinqDataSource1" runat="server" 
        AutoPage="true" AutoSort="true"
        ContextTypeName="JQSuiteASPNETExample.dbml.NorthWindDataClassesDataContext" 
        Select="new (CustomerID, CompanyName, Phone, PostalCode, City)" 
        TableName="Customers">
    </asp:LinqDataSource>   
    
    
    <br /><br />
    <trirand:codetabs runat="server" id="tabs"></trirand:codetabs>  
    
    </form>
</body>
</html>
using System;
using System.Collections.Generic;
using System.Linq;
using System.Web;
using System.Web.UI;
using System.Web.UI.WebControls;
using System.Web.UI.HtmlControls;

namespace JQSuiteASPNETExample.examples.functionality.localization
{
    public partial class _default : System.Web.UI.Page
    {
        protected override void OnInit(EventArgs e)
        {
            base.OnInit(e);
            LocaleRadioList.SelectedIndexChanged += new EventHandler(LocaleRadioList_SelectedIndexChanged);
        }

        void LocaleRadioList_SelectedIndexChanged(object sender, EventArgs e)
        {
            string js = LocaleRadioList.SelectedValue;
            localeLiteral.Text = "";
            if (LocaleRadioList.SelectedItem.Text == "Hebrew" || LocaleRadioList.SelectedItem.Text == "Persian")
            {
                JQGrid1.AppearanceSettings.RightToLeft = true;
            }
            else
            {
                JQGrid1.AppearanceSettings.RightToLeft = false;
            }
        }

        void DropDownList1_SelectedIndexChanged(object sender, EventArgs e)
        {
            
        }

        protected void Page_Load(object sender, EventArgs e)
        {            
        }
    }
}
Switch theme:


Theming is based on the very popular jQuery ThemeRoller standard. This is the same theming mechanism used by jQuery UI and is now a de-facto standard for jQuery based components. The benefits of using ThemeRoller are huge. We can offer a big set of ready to use themes created by professional designers, including Windows-like themes (Redmond), Apple-like theme (Cupertino), etc. In addition to that any jQuery UI controls on the same page will pick the same theme.

Last, but not least, you can always roll your own ThemeRoller theme, using the superb Theme Editor

To use a theme, simply reference 2 Css files in your Html <head> section - the ThemeRoller theme you wish to use, and the jqGrid own ThemeRoller Css file. For example (Redmond theme):

<link rel="stylesheet" type="text/css" media="screen" href="/themes/redmond/jquery-ui-1.8.2.custom.css" />
<link rel="stylesheet" type="text/css" media="screen" href="/themes/ui.jqgrid.css" />