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Set localized defaults for remote config

I am implementing Remote Config in my app. The idea is to be able to A/B test some of the messages shown to the user.
I might want to run some of the experiments with the English messages only and I am trying to figure out what is the best (or most practical) way to do this.
One way would be having an xml file with default values for each language in the res/xml- directory. The main drawback I see is that this will be hard to maintain as every single value needs to be copied to all the xml files (some experiments might be language independent such as a layout color).
A second way would be setting language dependent values to an empty string and then implement some logic to look for the right string in R.strings if the value provided by remote config is empty. This seems like too much overhead for what I want to accomplish.
What is the recommended way to run experiments only for a certain language with A/B testing and Remote Config?

It is a default way in android to provide a localized string resources. Is hard to maintain but there are a service which can help you https://phraseapp.com/ . They have a lot of nice features, like manual translation, cross platform app and ide plugins which can replace/add/append/remove new strings to your xml localized files, also it works for ios.
You can keep string resources name in remote config and load it by some kid of reflection. Here is an nice answer How to get a resource id with a known resource name? . Here is an example for your case:
int resourceId = context.getResources().getIdentifier(nameFromRemoteConfig, "string", context.getPackageName())
Localization a very static logic, boring and expensive process in terms of time and maintaining. From my point of view better solution is to develop your own maximum abstract mechanism for localization and reuse it in all your projects. For example a backend service or library which will provide localized data by your own format, and a library for clients which will deserialize that data. As a result you will obtain production ready api for future projects.

Multiple Language Support Android APP

I am very new for developing the multi-language oriented Android APP. This is the first time i am going to support my app in multi-support language app. I need some clarification for developing the multi-language support app. Already i have some ideas before initiate the development.
Single APK with Localization like will make the multiple String.xml and include it in inside of the app and based the member selection of language its automatically invokes and works.
All language strings values will retain in app server (back-end) and will raise the Rest-API request and get those values and change it inside of the entire app (all screens) if member click and change the language inside of the app.
Main concern is if anything needs to change in future then Idea2 is best way. We will just change in back-end side it will be appeared in client side. But if we are going Idea1 and wants to change then we need to put the new build only.
Which one is best approach and recommended way to develop?

You will need to create different String.xml depending the languages you want to target as JDenais says, in my app i have the following
for example the first arab string consist in the same as Strings.xml but with all in arab, now, you only need to call one string in your xml files and it will just select where to grab depending on the phone language. Or in default the language from where the app was downloaded by google play, in fact, they are all the same strings.xml , so you dont need to specify which one you want to pull the translated text from, just replicate your main strings.xml in your other strings.xml and then the phone will decide where to pull the data.
Also please read the official doc on how to accomplish this https://developer.android.com/training/basics/supporting-devices/languages
Also please check the language ISO Codes here
What is the list of supported languages/locales on Android?

All your texts should be packaged in the APK in different String.xml files. Forget about receiving texts from a backend. It would be a lot of extra work for limited gain and with added risks of failure.
The framework takes care of selecting the appropriate string.xml file and offers support for needed use cases like plurals.

right click the values folder and choose new-> values resource file -> locale -> choose the language you want and name it strings.
copy the strings from the original string file to the new file and change them to the new language just the strings
make sure that the view xml files have their text set as "#string/the_name of the string" not hard coded

How should I store API URLs in Android, if they are dependent on locale?

Right now they are just stored on a class called Globals as static fields.
I'm not the one who made the app, but I am considering putting them in localized strings.xml files, such as <string name="API_URL">http://someurl</string>. Is this good or bad practice?
UPDATE:
I chose the answer that I feel answers the question most comprehensibly.
But after some re-thinking, I have chosen a different solution alltogether.
Given that URLs are actually based on the country which the app should be distributed for, it doesn't make sense to switch them based on locale, as the URLs should stay the same regardless of the language on the phone.
Instead, I have implemented Gradle Flavors, which create different APKs based on different settings and such. It allows you to create variations of the same app with the small changes that you need. :) So now I have the URLs in a flavor-specific file.
Thank you to everyone who took their time to comment and help me.

I agree with puneet, it's neither good nor bad. It depends on what you are doing with the API Urls.
Are you going to append them later with user input? If so I would suggest you keep them as global variables that way you can modify the API URL programatically as needed.
If the API Url are complete and will not need to be appended then putting them in the strings.xml would be fine. Just remember that you would still have to create a local String variable in the java to hold the text from the API_URL in the string.xml, which seems inefficient if what you're aiming for is to write less code.

Neither good nor bad.If your concern is the security then none of them provide the security as decompilation is possible.

Why app ids are stored in strings.xml in Android?

Isn't strings.xml is supposed to be used as a text storage for easing the translation of text in the app to other languages?
For example - Facebook app id according to facebook manuals is advised to be stored in strings.xml.
It means that if I want to share this file with 3-th parties for translation - I will have to manually remove all ids by myself, or share those ids with 3-th parties.

Isn't strings.xml is supposed to be used as a text storage for easing the translation of text in the app to other languages?
No. It's string storage for any kind of strings. Majority of use is localization related but it is perfectly fine to have anything that is string there like API keys, tokens whatever.
Please be aware that you are not limited to just strings.xml file. You can have as many *.xml files holding string resources as you like (so it's quite common to split localization per class/functional module and keep it in separate xml file).

You can create more then one strings.xml you could name it appids.xml and store all your ids inside this file. It is common software design pardigm to seperate data from code, so you won't just use a String constant for your id.
For Android best practices, Google is the source to go, for example they propose the usage of an appids.xml file here: Getting started with Play Games
Facebook might be using this simpler form of storing your id to make the tutorial easier to follow.

Basically it may happen that the ids that you're going to use inside your app may occur in multiple java class files. So by mistake there may be a chance that you mistype the id or secret key which will result in failure of result that you are expecting. As a good practice you should store such things in strings.xml which will help you minimizing the possibility of error in your result. Also if you change your id or key because of any reason then you might have to change that in each file where you've mentioned it. Instead of that if you just change it in strings.xml then it will automatically reflect at every instance where you've used it.
Going further, Android is open source. Thus any app that you create can be reverse engineered and all the code can be read. This leads to leakage of your id's and may be some secret keys for any api that you've used inside your app.

Configuration file in Android. Does that exist?

I have some configuration I want to save it in my Android application and read it whenever I need , for instance, the server URL that it should try to access like that.
Is there any similar mechanism like web.config in ASP.NET available in Android?
A central configuration file that can be set up manually and then read by the application? Any help would be appreciated!

We use a .properties file in assets folder. It works out very well for us as we support multiple carriers with this, write to it (in case some values, sent from server, need to change. This is done at app start time, thus making our code configurable from server).

You can throw things like that into your strings.xml file. But, since you can't actually modify these values in real-time (since it's a distributed application rather than running on a server), throwing it into a constants class is quite acceptable.

Use Shared Preferences.
Here's a link Shared Preferences

You can use sq lite database files for it. You have a native API to read and write those and on top of that a command line tool.
If you want to create an XML file instead, then it's no different than any other xml file (unless you are thinking about the Shared Preferences, which use an xml format to save the data, but I believe it's not the best API for your application).

I was stumped on this too, but came across Managed Configurations in the Android documentation.
Managed configurations, previously known as application restrictions, allow the enterprise administrator to remotely specify settings for apps. This capability is particularly useful for enterprise-approved apps deployed to a managed profile.
It allows you to set a default value in case you rather not getting into the enterprise admistration business but leaves that option open for the future.
There is a caveat. This only works if your app is registered for EMM. Otherwise you will retrieve an empty map of restrictions.