Still no official answer, but XGtE makes it more likely
Xanathar's Guide to Everything, released Nov 2017 (five months after this question was asked and answered), contains the optional rule to allow characters to use a reaction to identify a spell as it is being cast, or to use an action to identify a spell by effect after it is cast. For the purposes of this question, the reaction is more important, for while identifying a spell after it is cast would rely on recognizing the effects of the spell, which are universal, identifying a spell as it is being cast argues strongly for universal verbal and somatic components that can, in fact, be recognized.
Xanathar's Guide to Everything p.85 (emphases mine)
Identifying A Spell
Sometimes a character wants to identify a spell that someone else is casting or that was already cast. To do so, a character can use their reaction to identify a spell as it's being cast, or they can use an action on their turn to identify a spell by its effect after it is cast.
If the character perceived the casting, the spell's effect, or both, the character can make an Intelligence (Arcana) check with the reaction or action. The DC equals 15 + the spell's level. If the spell is cast as a class spell and the character is a member of that class, the check is made with advantage. For example, if the spellcaster casts a spell as a cleric, another cleric has advantage on the check to identify the spell. Some spells aren't associated with any class when they're cast, such as when a monster uses its Innate Spellcasting trait.
This Intelligence (Arcana) check represents the fact that identifying a spell requires a quick mind and familiarity with the theory and practice of casting. This is true even for a character whose spellcasting ability is Wisdom or Charisma. Being able to cast spells doesn't by itself make you adept at deducing exactly what others are doing when they cast their spells
While none of this explicitly says that verbal and somatic components are the same for everyone, several details argue for that position.
Perceiving someone in the process of casting a spell gives you a chance to identify it. If another caster had their own repertoire of sounds and gestures with which you were not familiar, what would there be for you to recognize? Note also that this is pitched at a DC15, a "Medium" check, not even a hard one.
If the spell is on your spell list, you have advantage on the check. That is, you are more likely to recognize spells which you yourself can cast. That makes sense if what you are actually recognizing is the same sounds and gestures you yourself use. With the effective '+5 of advantage', recognizing spells you yourself can cast is essentially a DC10 "easy" check.
The rules say you can use this if you are "perceiving" a spell - not seeing it. This makes it likely that you are not only recognizing the universal gestures but the sounds as well.
The skill used is specifically Intelligence (Arcana). In this case, Intelligence is about memory, pattern recognition, and attention to detail. That makes sense if you are recognizing the familiar elements of universally used sounds and gestures. If it was more about understanding the intuitive 'feel' of the magic irrespective of different sounds and gestures, Wisdom (Perception) would be a better skill check, or Wisdom (Religion) for divine casters.
Thus, while not saying so explicitly, everything in these optional rules points to there being universal verbal and somatic components, just as we already know there are universal material components.
To answer your questions specifically:
is there any official statement that says that a spell has the same words/gestures independent of who's casting it?
Officially no, but the optional rules in XGtE make is seem very likely.
if a halfling wizard casts fireball: would the words he has to say as verbal components and the gestures he does as somatic components be the same as an elf sorcerer casting the same spell?
It appears so (with the caveat that the sorcerer may be using subtle spell metamagic so as to not use verbal or somatic components).
Would a ranger know that another ranger that he can see cast Hunter's Mark on him just by seeing the somatic components and/or hearing the verbal components?
Yes, provided he made an Intelligence (Arcana) check at DC16 with advantage and spent his reaction to do so.