Tip from a non-native English speaker: if you want your website to be more friendly to an international audience, don't use the terms "sign in" and "sign up", use more distinct terms (like "login" and "register") instead.
Phrasal verbs, in general, are difficult to speakers of languages that don't have them, especially when the same verb has different meanings depending on the added preposition. Someone with a basic/intermediate level of English may have difficulty telling between "sign in" and "sign up". In my own case, I have a good level of English so I know what they mean, but "sign in" and "sign up" always take 2 or 3 seconds for me to disambiguate, while "login" and "register" (or similar) are instantaneuous.
Phrasal verbs, in general, are difficult to speakers of languages that don't have them, especially when the same verb has different meanings depending on the added preposition. Someone with a basic/intermediate level of English may have difficulty telling between "sign in" and "sign up". In my own case, I have a good level of English so I know what they mean, but "sign in" and "sign up" always take 2 or 3 seconds for me to disambiguate, while "login" and "register" (or similar) are instantaneuous.