Python perhaps. Not wanting to start a programming-language-religious-war but I consider C to be a primitive language. If you are writing device drivers or kernel code, C could be good but …
As a freed environment, you are of course free to put whatever software on your phone you like, but I am not sure that there will be Java support out of the box at Day 1 on the Librem 5.
Happy to stand corrected on that.
Will the JRE be there? Will it feature JIT compilation? (On the Raspberry Pi the speed difference between the JIT and non-JIT versions is stark.)
The previous discussion also raised issues about the Java bindings. In other words, if what you want to do can be done without Java bindings then it may work better than if you need to go outside the Java environment.