No problem.
About the Letter of Explanation;
It is mandatory and you could be denied just for not adding it to your documents when submitting. It should contain why you are going to do that particular program, why you chose Canada and not any other country, why you chose that university, what you intend to do with the program when you are done etc. You can also explain certain details that were not captured in your application form, like your accomodation, your expense etc. Note: letter of explanation is not a descriptive essay about you, since the visa office will have that informatiin from your application form.
At the Employment/Occupation/Activity slot enter what you were doing during the period you weren't working.This part is also compulsory, never leave it blank. Even if you aren't doing anything at all, get something to tell them. For your case you have to enter that you are a "student" from say 1/3/2015 (from when you started) to 5/3/2018(when you finished; if you are still studying, then leave this side blank), since you were learning English and Germany. Don't forget to enter the name of the institution or body where you were studying these languages. Now your previous employment, which will be the second enlisting will again be "student" since you were in school, state the duration of studies and the name of the school. That should be enough for yours concerning the employment.
Finally, "Personal Reference Code" is for those who want to come back to Canada to work, express entry etc.. so dont worry about that since you are a first-time applicant. You can follow this link and see for yourself:
http://www.cic.gc.ca/english/helpcentre/answer.asp?qnum=830&top=23
Also about the personal eligibility question, its mandatory for every online applicant. So don't stress yourself.
Cheers!