What you should do is to build my solution from scratch.
Make build/ directory in src/, move resources/ to src/, so that folder has the appropriate structure:
src:
-- CMakeLists.txt
-- tglang.cpp
-- tglang.h
resources:
-- languages.txt
-- libfasttext.a
-- model.bin
fasttext:
/* FastText source code files. */
build:
/* You are here now. */

Then, run:
cmake -DCMAKE_BUILD_TYPE=Release ..
cmake --build .

Now, resources/ can be moved to libtglang-tester/build/, so the structure is:
CMakeLists.txt
libtglang.so
tglang.h
tglang-tester.c
build:
-- resources:
---- languages.txt
---- libfasttext.a
---- model.bin

After that, you can build tglang-tester by running the same commands:
cmake -DCMAKE_BUILD_TYPE=Release ..
cmake --build .

Everything should be OK now.
Good luck.

In our final tests, this submission received the following scores (out of 100):

Code/Other Detection: 20.9
Language Detection: 50.3
Average Time: 190ms per sample

Final Score: 8

This is an important message regarding your submission. The following issues have been discovered during preliminary testing:
"/lib/x86_64-linux-gnu/libm.so.6: version `GLIBC_2.29' not found (required by ./libtglang.so)
/lib/x86_64-linux-gnu/libc.so.6: version `GLIBC_2.34' not found (required by ./libtglang.so)
/lib/x86_64-linux-gnu/libc.so.6: version `GLIBC_2.32' not found (required by ./libtglang.so)
/lib/x86_64-linux-gnu/libstdc++.so.6: version `CXXABI_1.3.13' not found (required by ./libtglang.so)
/lib/x86_64-linux-gnu/libstdc++.so.6: version `GLIBCXX_3.4.29' not found (required by ./libtglang.so)
/lib/x86_64-linux-gnu/libstdc++.so.6: version `GLIBCXX_3.4.26' not found (required by ./libtglang.so)"

You can provide further clarifications within the next 24 hours.