In portuguese, there's a lot of people who insist on using "mais" (plus, more) instead of "mas" (but). How you speak it ends up being nearly identical, so that's the reason, much like the there/their/they're in english.

I don't care and usually let people speak the way they like but when my girlfriend says 'voy a subir arriba' I always tell her that you can't 'subir abajo' just to annoy her.

In Turkish de/da can be a suffix or a conjunction or of course a part of a word. If de/da is used as a conjunction you have to write them separately. If it's not written correctly it can be confusing for those who are reading the sentence.

Example 1:

"Bende gittim" instead of "ben de gittim." (I've gone too). "Ben de gittim" is the correct sentence. De means too in this example.

Example 2:

O da iyi (It is good too). "Da" means "too" in this sentence. Oda iyi (The room is good). "Oda" means "the room". Odada iyi (It is good in the room). 2nd "da" means "in". Oda da iyi. (The room is good too). 2nd "da" means too. Odada da iyi. ( It is good in the room too). 2nd "da" means "in", 3rd "da" means "too".

In Spanish to English translation with Google and others, need often to be corrected manually.

native speakers cannot by definition make systematic errors. they cannot make "common mistakes". if a thing is common, that's the correct way to say it. so what do you mean? spelling mistakes? (spelling is a separate thing from language)