Roni and Carlos

Yeah, he looks like he's under 21 in the OP's photo.  

Father sponsors the son.   I-864 they want you to cosponsor for the family (unmarried son) F4 visa right?   Technically you dont have to.   Father or brothers or other aunts or uncle family is better in my opinion, as like I mentioned if things go south, you are not off the hook for I-864 easy.  Imagine the scenario of your bf gets the gc , then something happened and either has a change of heart.  Now he is not even your husband and you are stuck with a I-864 potential liability if he becomes a public charge -  FOR LIFE pending the 4 exit conditions.
 
If things are ok, no problem.  I am just giving you scenarios to consider - of course in my heart I wish you a successful marriage.

Agree with @Hank_.  USPS Priority Mail is the way to go, imo.

Use PRIORITY MAIL ... has tracking just like the "big boys"  (which can't be used because it is a PO box)
 
The USCIS office in CA is over by Disneyland, in fact there is a Disney parking lot across the street ... still I don't know the street address.    
 
 

You send it to the PO box, the use of any other address will delay your reply 

I would send it to the address it tells you to send it. I sent it back certified mail so I was able to track it.

I received literally this exact same RFE for our I-129F.  I checked 'No' in the area provided on the second page of the RFE, signed and printed my name where indicated, and returned it.  We were approved after that, no problem.
 
The language in the first page is a form letter they send with all RFEs, the second page is specific to your case.

Per page 11 of the instructions it says the name should be printed with pencil or light felt pen. Me and my fiancé both signed our names and dated it on the back. If you go to the USCIS website, click forms, select I-129F and then click form details it will show the actual I-129F form and instructions underneath. I’d read through that thoroughly.

I was advised that all letters of intent should be original and signed individually.  So, they told me I should not used copied or scanned letters.  I've never head of having to sign the back of the 2x2 picture (the passport size picture) per the instructions I followed when preparing my original packet.

I didn't translate any. In fact, I think sending our messages in Spanish is further proof of ability to communicate. I'm pretty sure they didn't even look at ours though. They certainly didn't ask to see any more in the interview.

I sent my text-snapchat- all in spanish. Like the others already told you. You should be fine! 

Plane tickets along with pictures and passport stamps should show you met.  If you have credit card or bank statements showing you spent money there can also help.

***Moved from Progress Reports to Process & Procedures.***
 
**Moderator hat off**
 
If you were to decide to submit the texts as evidence (not proof) of a relationship, you do not have to translate them, there is no requirement to translate such evidence.

Proof of what? All they prove is that you have mobile devices and can read and write. 
 
You dont hace to submit “proof” of anything with the I-129F other than you are a USC (if you are the petitioner), that you have met face-to-face in the last two years and you are free to marry each other. 

You can't provide what you don't have 😉 So yeah, you're fine. Also send copies of the passport pages showing the stamps from entering the country to visit.

Yup, but you can translate them yourself IF you're fluent in Spanish and English. You also have to write a statement that you're fluent in both.
Don't send too many texts, since they're not hard proof you met in the last 2 years. Pictures, plane tickets, hotel bookings etc are more important in this proces.