Example mod for Here Comes Niko!

Main.cs

using UnityModManagerNet;
using HarmonyLib;
using System;
using System.Reflection;

namespace NikoOwO
{
    static class Main
    {
        public static bool IsEnabled;

        static bool Load(UnityModManager.ModEntry modEntry)
        {
            var harmony = new Harmony(modEntry.Info.Id);
            harmony.PatchAll(Assembly.GetExecutingAssembly());
            modEntry.OnToggle = OnToggle;
            return true;
        }

        static bool OnToggle(UnityModManager.ModEntry modEntry, bool value)
        {
            IsEnabled = value;
            return true;
        }

        [HarmonyPatch(typeof(LocalizationManager), "GetLocalizedValue", new System.Type[] { typeof(string) })]
        static class LocalizationManager_GetLocalizedValue_Patch
        {
            static void Postfix(string key, ref string __result)
            {
                if (!IsEnabled)
                    return;
                __result = Owoize(__result);
            }
        }

        [HarmonyPatch(typeof(LocalizationManager), "GetLocalizedValueOrEmpty", new System.Type[] { typeof(string) })]
        static class LocalizationManager_GetLocalizedValueOrEmpty_Patch
        {
            static void Postfix(ref string __result)
            {
                if (!IsEnabled)
                    return;
                __result = Owoize(__result);
            }
        }

        public static string Owoize(string v)
        {
            var x = v.Replace(";", "; @!@ ").Split(' ');
            for (int i = 0; i < x.Length; i++)
            {
                if (x[i].StartsWith("##") || x[i].EndsWith(";")) continue;
                x[i] = OwoizeInt(x[i]);
            }
            return string.Join(" ", x).Replace("; @!@ ", ";");
        }

        public static string OwoizeInt(string v)
        {
            return v.Replace("r", "w")
                    .Replace("R", "W")
                    .Replace("l", "w")
                    .Replace("L", "W")
                    .Replace("na", "nya")
                    .Replace("ne", "nye")
                    .Replace("ni", "nyi")
                    .Replace("no", "nyo")
                    .Replace("nu", "nyu")
                    .Replace("Na", "Nya")
                    .Replace("Ne", "Nye")
                    .Replace("Ni", "Nyi")
                    .Replace("No", "Nyo")
                    .Replace("Nu", "Nyu")
                    .Replace("NA", "NYA")
                    .Replace("NE", "NYE")
                    .Replace("NI", "NYI")
                    .Replace("NO", "NYO")
                    .Replace("NU", "NYU")
                    .Replace("ove", "uv");
        }
    }
}