It depends on the connotation you want to give:
I am lonely (because I chose to be alone): Soy solitaria/o (female/male)
I am lonely (because nobody comes to see me): Me siento solo.
(Yo) estoy soltero sin ti
lonely girl = chica/nina solitaria
Todos los solitarios.Toda la gente solitaria. Todas las personas solitarias.
solitario
This is probably the way to say lonely in Russia одинокийGlad for answering =]-Alexwikihelper
Solo is 'singly' or 'alone', thus one might say that one, "siento solo" or 'feels alone'. The word which most closely translates directly is, "solitario".
Lonely is "seul / seule" or "solitaire" in French.
LAGRIMAS SOLITARIAS
To say I'm bored and lonely in French, you say "Je suis ennuyé(e) et solitaire"
Devia.
There are 42 less lonely girls in Justin Bieber's song ''One Less Lonely Girl''.
la fille solitaire