It's a challengeing for me to learn two languages at the same time.

Sometimes it is hard to find time or motivation but in the end it is really fun.

It' is a challenge for me to learn two languages at the same time.

In English, we use the infinitival particle, the word "to", before a verb to talk about opinions and feelings like this. Here is a website that shows examples to help understand how to use the word "to": https://learnenglish.britishcouncil.org/english-grammar-reference/infinitives

Also, when were are comparing two things, like languages, we use the word "the" when we talk about learning "two languages at the same time".

Finally, I would stay away from using the contraction "it's" for right now. It is very informal and gives a sense of being either familiar with the person you are communicating with or of a fairly young age. Children and teenagers tend to use contractions more than adults, at least than adults that are fairly educated.

Sometimes it is hard to find time or motivation, but in the end it is really fun.

Both instances of your word "is" are being used to describe the state of existence. For this type of "is", we use the word "it" with it, which ends up either being "it is" or "it's" (in a contracted form). Also, when you use "hard" as an adverb, rather than an adjective, you need to include the word "to" after it but before the verb.

It's a challenge for me to learn two languages at same time.

Sometimes it's hard to find time or motivation but in the end it's really fun.

In the end it's worth it