"Ever since I learned German" is a dependent clause, and thus must be separated from the independent clause by a comma.

Actually, it's adverbial introductory phrase that acts as preposition with the clause "I learned German" as the object of "ever since."

You can rewrite the first part of his sentence as "I have no idea where commas are supposed to go in either language ever since I learned German," and it retains its meaning.

Also, you're wrong about his second clause as well. It's an independent clause that is linked by the coordinating conjunction "so." If it were dependent, it would need to have incomplete information (e.g. an unanswered interrogative) somewhere.

It may be awkwardly phrased, but you could rewrite his statement as two sentences:

"Ever since I learned German, I have no idea where commas are supposed to go in either language. So I just don't use any most of the time."

You might anger a few curmudgeons out there, but there's nothing necessarily wrong about beginning a sentence with a coordinating conjunction.

Honestly, most of the time in English, if you were to read the sentence aloud and paused at a certain place, put a comma there. I'm sure there will be lots of people who will come along and tell me all the places this doesn't work, but I'm going to say 80% of the time this functions.

If you're speaking about standard formal English, you really are giving bad advice. Basing where you put a comma on verbal pauses is how you get awkward comma splices and angry English Comp. teachers. It's a nice rule of thumb on occasion, but I've seen too many people try to add "rhythm" to their formal writing with commas. Just learn when and where to use them if you're concerned about it.

http://www.quickanddirtytips.com/education/grammar/where-do-i-use-commas

It's also easier to just say "whenever you'd pause" when you have an intuitive grasp on spoken English. For a non-native English speaker, this is not always the case.