My dad wrote a letter in Russian, I translated it, he printed, signed and mailed it to us.
My experience with this is similar. Yes, the guide says all documents must be translated by certified translators, etc. And absolutely (I repeat: absolutely) this must be done for ALL official documents of any kind (and according to local regulations notarised if necessary, etc).
I warn I'm not a lawyer and at your own risk, etc.
This does NOT in my view mean that every document must be written in original language and translated / certified by certified translators.
For example, in your case above, your father signed an English language document. As long as he understands the content and the content is all true, that's fine. He signed it.
What I did in most of these cases - made more easy that most of my friends and others that signed such letters (e.g. landlord) speak and read enough English, even if they wouldn't be comfortable writing a letter in English - is that I drafted letters, asked them to read and if they were okay with content, to sign. (A few made minor comments). But if e.g. your dad trusts you and the content is true, he can sign it in English.
Also: keep these documents / letters short and factual. Your friends and relatives do NOT need to opine about how much you love each other, etc. None of that. For the most part, this is: I have known the parties since [date.] I know they have been living together/a couple since [date]. I know they have been living together at [address] since [date]; I know this because I have visited them in their apartment [every week since] [date]. They are known as a couple by friends and relatives. I attended their [wedding / engagement party / christening / etc.] Factual stuff is more important than poetry.
The term 'affidavit' is thrown around a lot. At core, an affidavit is just a letter or written statement saying so-and-so said this in front of me, I attest to it, or even an individual writing I swear this is true - again, it's just a written statement. In court contexts, this means the person swearing it can be held to account if the statements are not true (e.g. perjury) so the affidavit is often done by an officer of the court (e.g. a lawyer) or notary or the like. As part of the application package, you already affirm that everything in it is true and the 'held to account' aspect is that your application can be refused if parts are not true. The goverment of Canada is not going to go after e.g. your father for making false statements, they'll just deny the application. In short, the letter from sponsor, or applicant, or friends, or landlord, or whatever is basically an affidavit - it's just a written statement. Most of the time getting a lawyer to sign off is just a waste of money.
Note, theoretically IRCC could contact the author of the letter and it might lead to confusion if that person doesn't speak English. But they don't often do this; even if they do, if the content is true, there's no issue (just might require translators/interpreters to confirm); and perhaps most importantly, if you provide letters and IRCC still doesn't believe you, it's probably not because one letter was a bit off or wasn't certified by a lawyer or translator, but bigger problems with the application.
Note, notice mostly I referred to getting documents signed that are done in English or French. For things like chats that were in some other language between the applicants, probably does need to be translated by a third party.
Caveat internet, at your own risk, etc. I submitted docs like this, no problems so far. But it will also depend how problematic or simple (how many red flags) the application is. For a straightforward application, they're less likely to worry about this sort of thing.
It is possible IRCC could return if they don't like the translations - and they absolutely WILL return official docs that don't have official, certified translations. But there are also ways to provide (truthful) docs and attestations from family etc that don't mean spending lots of additioanl money on lawyers and official translators.
(Of course some may prefer to get official translations, by all means, go ahead if you can afford it and it's easier)