Short instruction:
Installation:
Extract the archive file and place the files from the "content" folder in
C:\Users\your_account\Documents\Planetbase\Challenges

Detailed instructions:

1) first, you must have a Challenge file that you want to load into the game.
If your file is in the form of an archive, then unzip it.The file archive usually contains both the Challenge files themselves and additional files.
For example, if the file is like "ReadMe.txt" or "*.jpeg" then this is informational material that is not used by the game.

The Challenge consists of at least two files:
1.1) "challenge_NameChallenge.xml" is the main file with mission parameters.
1.2) "en_NameChallenge.xml" - a file with text information, mission description.
Additionally, more files can be added:
1.3) If other languages ​​besides English are provided in your Challenge, then there should be additional files for each additional language. 
For example, for the Russian language there will be a file of the "ru_NameChallenge.xml" type.
1.4) Challenges which begin not with a rocket landing, but when there is already life on the planet and something exists, they will additionally have a game save file of the form: "challenge_NameChallenge_save.xml".

2) specify which first 2 letters are in the name of the file with the Challenge text information.
Look in the files of the selected Challenge how the file with text information looks there.
The file with the text information looks like "en_NameChallenge.xml".
Where "NameChallenge" is the name of the Challenge (Mission).
It is important to understand what 2 letters are at the beginning of the file.

If the letters "en" are at the beginning, then your Planetbase game will be able to perceive the text information of the Challenge only if the settings are set to English.

If the letters "ru" are at the beginning (ie a file of the form "ru_NameChallenge.xml"), then your Planetbase game will be able to perceive the text information of the Challenge only if the settings are set to Russian.
If the set of Challenge files includes several files of text information at once, then the creator of the Challenge has provided that his Challenge can be downloaded by people from different countries.

3) make sure that the file with text information has a file name that matches your language settings in the game.

4) you can change the first 2 letters in the file name to those you need for a comfortable game.
Rename the file, replacing "en" with your two letter ISO code. 
For example, for the Russian language it will be “ru”, and a file with a name like “ru_NameChallenge.xml”.
If the game does not accept the file with the text information about the Challenge, the Challenge will start anyway. 
But then you will not be able to read the description of Challenge and the tasks for her.

5) specify in what language the text of the Challenge information inside the file.
Open the "ru_NameChallenge.xml" file in a text editor and read what is written there. 
Even if the file name contains the letters "ru", there will not necessarily be a description in Russian inside. And vice versa.

6) If necessary, you can translate the text inside the file and replace it with the language you need.

7) If you overwrite the text of the file, make sure you save the xml in UTF8 format. 
Otherwise, inside the game, the text may be reflected by artifacts.

8) put all Challenge files in "% UserProfile% / Documents / Planetbase / Challenges".
Please note that this is not the path where the game is located. 
The game can be located anywhere. And even you can have many different PlanetBase games installed in different places on your hard drive. 
For example, in different versions of the game.
All of your PlanetBase games, when starting the game, will be asked to download additional Challenge to this "Challenges" directory. 
The Challenges directory is located in your operating system account, and the game itself will most likely be installed in a different location on your hard drive.

9) Now the new Challenge should appear in the Challenge selection menu in the game. 
The loaded Challenges hould appear at the bottom of the built-in (initial) Challenge list, right below them. 
Scroll the slider or mouse wheel down to move down the list of Missions.

10) Click on the selected mission and it will start.
Great game!
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GOG-Instruction for working with additional Missions (Challenges) that are not in the Steam Workshop: (GOG, Mackam)
https://www.gog.com/forum/planetbase/laplacian_sector_halfway_there_new_challenge_for_the_gog_version_of_planetbase

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Useful information about the creation of Challenges (Missions, Companies, Maps) for the game Planetbase:

1) "Creating Challenges"- the official guide from the developers of the game: https://steamcommunity.com/sharedfiles/filedetails/?id=677895200
 
2) "Blank XML for challenges with instructions" https://steamcommunity.com/app/403190/discussions/0/2119355556483427227/

3) "Save file editing" https://steamcommunity.com/sharedfiles/filedetails/?id=537016058

4) "Creating Planetbase challanges" (from Base Invaders) https://steamcommunity.com/sharedfiles/filedetails/?id=1955921323

5) "Create an island map for challenge" https://steamcommunity.com/sharedfiles/filedetails/?id=1987002995

6) [Rus] "How to create a mission (challenge) with an unusual map?" https://steamcommunity.com/sharedfiles/filedetails/?id=1983521727

7) [Rus] Alignment of landscapes with a cheat mod https://steamcommunity.com/sharedfiles/filedetails/?id=1983467709

8) [Rus] F.A.Q. Challenges (Missions \ Companies) - All About Challenges https://steamcommunity.com/sharedfiles/filedetails/?id=1979452591

9) [Eng] Guide in English with the description of the program "Planetbase Challenge (Mission) Editor (PBCE)": https://steamcommunity.com/sharedfiles/filedetails/?id=1991143520

10) [Rus] Guide in Russian with the description of the program "Planetbase Challenge (Mission) Editor (PBCE)": https://steamcommunity.com/sharedfiles/filedetails/?id=1983563107


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A full LIST OF GAMES similar to the game "Planetbase":

A full list of games similar to the game "Planetbase" you can see here:
1) Guide "Space colonization & survival Games" - https://steamcommunity.com/sharedfiles/filedetails/?id=1994216417

2) Group SpaceExperience in Steam: http://steamcommunity.com/groups/SpaceExperience
This group contains the largest and most structured list of games and programs about space.
Here you will find everything that exists to feel like in space, see VR space, build a colony,
feel the harsh conditions for survival, control a rocket and repair it, arrange a battle in space, see 3D maps of planets and universes,
and much other. List of strategies

3) Curator SpaceExperience of games about space in Steam: https://store.steampowered.com/curator/37277628-Space-Experience/
Reviews on all space games.
A huge number of lists and categories.
The most complete information both among those games that have already been released and among the new products.
List of strategies 
Here you can choose a space-themed game to your liking.
Also, here is information about all distributions of games and sales.

Enjoy!