"Sign up" translates to "enter your signature" on Google translate in some languages. Whereas register and log in seem officially supported in many languages.

Afaik, you "sign up" for something, like a newsletter, or health insurance. You sign into a hotel, ie leave your signature at the check-in (or sign-in) desk. Btw, also not a native speaker.

Native speaker here. You sign in when you arrive at a fitness club, or when you take your child to daycare. It's a regular, reoccurring action that you use to indicate you've arrived. I wouldn't normally use sign in for a hotel visit since it's not normally reoccurring; you check in to a hotel, not sign in. Sign in would be appropriate if the hotel had a policy of asking you to sign every time you enter or exit (to keep a minute to minute record of when you're in your room). I've never seen a hotel that does this though.

By the way, this is why an English speaker would say "sign in to a website" but not "check in to a website". Check in is generally a non reoccurring action.

A minorly relevant note is that GP said ‘sign into’ a hotel, while you said ‘sign in to’, which is a subtlety difference a native speaker would know.

Oh, I actually use Writefull to check phrases, this one came up in Google Books. (https://writefullapp.com/)

"after the raid, it is known that Brown wrote to Kagi that he would sign into a hotel as I. Smith and Sons. As he began recruiting supporters for an attack"

These are the results I get for web:

    - sign in to a hotel appears 0 time in Google.
    - sign into a hotel appears 81 times in Google.
    - check into a hotel appears 577,000 times in Google.
    - check in to a hotel appears 69 times in Google.

OK, that’s interesting. I wouldn’t really trust to google books since it is OCRed.

I can’t get Google results like those with numbers on mobile, as far as I know. What comes up for me is discussions about what is grammatically proper in this case. https://www.quora.com/Which-is-grammatically-correct-check-i... for instance. All I had to search for was the phrase “check into hotel“, and the results show this is a hot topic of grammatical discussion because that is what came up rather than information about hotels.

‘Check into’ sounds different than ‘sign into’ to me, probably because few say sign vs check for a hotel. I suppose grammatically they’re the same.

I would certainly flag it when editing. I also do not commonly see anyone use the contracted form for this. The reason is that the verb is the phrase “sign in”. The word ‘in’ is not a candidate to be modified by being combined because it is paired with ‘sign’. We would never modify a single-word English verb by adding a suffix like ‘to’, and the same rule applies for phrase verbs. Beyond that, I’m not a grammar expert, so I would imagine the above link could shed more light than I can.

Right, "check into the hotel" makes me think you're going to examine it, like you're suspicious of it's going to be a good one or something. Similar to how you might say "I'll check into that" if someone tells you to investigate something.

Thanks for the clarification, it makes sense.

Thanks for correcting me.

But log in is also a phrasal verb...

which is close enough to "login", which is widely understood (I do have the same issue with "sign in" VS "sign up", I know the difference, but as a non-native speaker it also takes me a second, or sometimes one is more prominent than the other and I mistakenly click on it, etc).

But whatever, ("Sign in" or "Log in") alongside "Register" is clear enough. "Sign in" alongside "Sign up" is confusing. Thanks Al-Khwarizmi for bringing that up... or is it bring in ? ;)