Question: I've got a situation where I want to put a wire in a concrete wall (I have a channel), but the one coming in isn't long enough. I cannot replace the incoming cable, so I need to join it with another one inside the channel. Said channel will afterwards be filled with cement, so I won't be able to access the joint anymore without some serious destruction. What is the best way to join the cable so that I can sleep easily knowing that there will be no problems there? If it matters, the cable will not be carrying significant loads (less than 100W max).

**Added:** Clarification was asked in comments. Location in world: Latvia. We're a 220V country. The incoming cable in question is marked "2x1,0" which (if I understand correctly) means two wires, each 1mm2 cross-section. Copper. It's pretty thin. The cable with which I will join it will be similar.

It will feed one LED lighting fixture (up to 50W, but probably less) and a wifi router. The location makes it unlikely that heavy loads would be attached there, however, of course, I cannot predict the future.

Answer: 
I doubt anyone here is familiar with code in Latvia.

That being said, the "correct" way to do this in most code-complying areas would be to leave the splice serviceable. One option in your case would be to carve a hole in your wall and install a box to splice the wires in, and leave the box cover accessible (do not cover in cement). If you don't want a box cover there, then another option is to trace you cable back to a place where a box would be acceptable, and run new cable from that point.

Some jurisdictions allow the use of splice kits that are designed to be inaccessible after installation.

If you don't know what is acceptable in your jurisdiction and you can't learn yourself, then you will need to hire a professional to either do it for you or educate you.
"cable in a concrete wall" is not a good idea, and this is why. No maintainability. Better to put the wires through **conduit**. This is one place where steel conduit is not good, and plastic works better.

With conduit, you can pull the wires out readily and at will, and then pull in replacement wires. The conduit must be built "to be able to be pulled" e.g. you can't have a plumbing elbow in there, it needs to be broad "sweeps" of turns. In North America conduit is required to be built empty of wires, and then wires pulled in after completion. This "keeps you honest" lol.