It is not mandatory to fill in all profile information. Your account will work fine even if you only provide your language skills. But if you want to fill that completion bar up to 100% and add a little meat to the bone, please make sure to provide the following:
- A profile picture
- A bit of info in the "About me" field
- Your profession, highest education and grade
- Your language skills
- Your Hobbies & Know-How
- A short (300 words max.) work sample
- A (personal) blog, if available