Both languages belong to the same family (Latin languages) but no, they are not the same. It looks like a mix of French and Spanish (and also Italian i think) but it's really a language on its own, not a dialect of Spanish/French.

I can easily think in both English and Russian (not the mix of 2 but rather separately) since I started to immerse into English area pretty early. And even though Russian is my first language and I studies in school and university in Russian, I was coming to Canada for visits quite often (my sister moved here long time before me)
I'm not sure if it is normal but currently I even have dreams in both Russian and English.

I would say, environment in which you were brought up is what matters...

Just because you, personally, only think in one language, doesn't mean that we all do.
I think in a multilingual mix.
Generally a mixture of my three first languages, with the occasional appearance of my two second languages ...meaning that I'd go from two native tongues, to... well should I just count the main three, or all five? I don't think in my mother tongue, as often as my other first languages. Does that mean that I shouldn't count it, despite having learned it from birth, and never having ceased using it?
Not particularly less confusing, quite frankly.
Quite the opposite.

"the language you consider your homeland/nation" is a bit closer to what a native language is, though it's not quite right.
It wouldn't change a thing for me, either way: I'd still have the same two native languages.

I'm no admin, but...

Native language:
"One of the more widely accepted definitions of a native speaker is someone who was born in a particular country and was raised to speak the language of that country during the critical period of their development"
-Wikipedia

"One's first language, learned in early childhood."
-Wiktionary

"the language of the country that someone is born in or native to"
-Collins English Dictionary

(actually entry for "Native tongue") "the native language of someone; the language of the country that someone is born in or native to"
Collins English Dictionary


Native (as an adjective):
"Belonging to one by birth."
"Born or grown in the region in which it lives or is found; not foreign or imported."
"Arising by birth; having an origin; born."
"Original; constituting the original substance of anything."
"Naturally related; cognate; connected (with)."
-Wiktionary

"Associated with the place or circumstances of a person’s birth"
"(Of a quality) belonging to a person’s character from birth; innate"
-Oxford dictionary

"living in a particular country, area, or city since birth"
"native abilities or qualities are those that you have had since birth"
Macmillan Dictionary
Native (as an adjective):
"Belonging to one by birth."
"Born or grown in the region in which it lives or is found; not foreign or imported."
"Arising by birth; having an origin; born."
"Original; constituting the original substance of anything."
"Naturally related; cognate; connected (with)."
-Wiktionary

"Associated with the place or circumstances of a person’s birth"
"(Of a quality) belonging to a person’s character from birth; innate"
-Oxford dictionary

"living in a particular country, area, or city since birth"
"native abilities or qualities are those that you have had since birth"
Macmillan Dictionary

Some dictionaries give a wider definition, like "language learned in early childhood", which is actually the definition of "first language", but that is merely a lazy/clumsy/ignorant dismissal of what it means for a language to be "native".
A mother tongue, however, is the language(s) of your parents. Of your people(s)/ethnicity/ethnicities.
You can have a person who has, say, Polish as their mother tongue, but who has never learned any Polish, and indeed doesn't know a word of it.

In short:
First language: A language before the age of 6-7, and which is still maintained.
Native language: A language you are born into, and learned from birth (more or less). By nationality and/or residency.
Mother tongue: The language of your parents/blood. No requirement of competence.