I'm having a problem with proving the following statement:
For every infinite language $L$, does there exists an infinite language $L' \subseteq L$ such that $L'$ is not decidable?
I'm having a problem with proving the following statement:
For every infinite language $L$, does there exists an infinite language $L' \subseteq L$ such that $L'$ is not decidable?
The set $\{L' : L' \subseteq L\}$ is an uncountable set of languages, and since there is only countably many decidable languages, it has to contain an undecidable language. (You can get a concrete $L'$ by a diagonal argument.)
Since $L'$ is totally free, you can always define it directly as something undecidable.
For example ordering the words of $L$ with lexicographic order and ordering turing machines (in any order you like) you can define $L'$ as: $L'=\{w_i|w_i\text{ ith word in $L$ and $M_i$ stop on }\epsilon\}$.
If $L'$ is decidable then termination with empty tape of Turing machine is decidable. Contradiction.
Hence $L'\subseteq L$ and $L'$ is undecidable.
Note I assumed $L$ decidable. If $L$ is already undecidable then take $L'=L$.