On a tangent (not related to cross-language solutions), there is also a latent problem accessing others’ solutions when the puzzles are too closely related.
For example it’s absolutely possible to solve the Mars Lander 1 problem with code from Mars Lander 3. I haven’t trawled through them, but I’m pretty sure it would be possible to solve Lander 1 legitimately (in just about every language) and find free solutions to Lander 3 there.
(it’ll happen in community puzzles too, eventually)
It’s indeed something I have been thinking about and it’s fun you are picking mars lander 3 as example…
I actually have been moving my mars lander 3 code to mars lander 2 in order to check how it was behaving there and thought about the possibility to submit it by mistake spoiling the challenge for anyone checking it.
Once you completed the problem, there is no much point checking other’s people code anyway.
If you are curious about how a particular person solved it, you can just directly ask them…
I don’t think this is a big problem. If you can do Mars lander 3 at 100%, you have +1000CP (just for Mars lander). So i think you are already at more than 5000CP. So copy/pasting some code to resolve Mars Lander 1 will not change your score that much.
If you want to learn a new langage, having functionnal code for a particular example is very valuable. And i can take for example the puzzle Triangulation. I have a 100% code in Javascript. There’s only 2 others solutions in javascript. And according to their code, our codes are very differents. So i suppose there’s many different solutions for this puzzles. But i can only see two of them 
I don’t think this is a big problem. If you can do Mars lander 3 at 100%, you have +1000CP (just for Mars lander). So i think you are already at more than 5000CP. So copy/pasting some code to resolve Mars Lander 1 will not change your score that much.
You read me backward.
Some players have solved ML3. The code they have to solve ML3 happens to solve ML1 as well. So it’s possible they submitted ML3 code to ML1, where it would be available to newbies.
So it’s possible that people who are not able to solve ML3 have access to a solution anyway.
Oh you are right, i misreaded your post. Well, i don’t know if this is good idea to try to handle this specific case.
Mars Lander 3 is not a good example because if you really want the points without to improve yourself, you can just hardcode it. And it’s not really harder than copy/pasting a solution.
The problem is the same with Indiana puzzles but again, you can just hardcode them.
I would like to vote for some improvement on this front as well. Being able to see solutions in other languages would be a huge growth opportunity for me.
I use codingame primarily for personal growth, not as some venue to brag over virtual points or achievements. If others want to cheat to gain such, what harm does it do to me?
In fact, not allowing to see all the solutions does only work for the two (if I remember well, one in bash, one in clojure) puzzles that have language achievement ; some puzzles have conditions achievements that still can be “cheated” by taking some code, or ideas, in the same language (for exemple, I admit I found new ideas by looking at other peeps code to unlock last “Triangulation” achievement). Why not just lock Clojure solutions for Roller Caster puzzle and Bash solutions for Horse-racing Duals ?
One weird thing I noticed is that when the puzzle is not yet solved, it is possible to see who solved in, when and in which language (even if the solutions are not accessible) ; when it is solved, this possibility does not exist anymore.