Foreign key question

New to foreign keys and have some questions…

The first is, i have a language table with a primary key on the fields
lang and relid:

relid | lang |    text
-------+------+-------------
  11111 | uk   | hello
  11111 | dk   | hej
  11111 | de   | guten tag
  11112 | uk   | world
  11112 | dk   | værld

In another table, texts, I have the following:

id | text
-------+------+
  4 | 11112   |

What I want to do is to create a foreign key between the two tables.
But trying to do so, with the following syntax, I get the following
error message:

 > ALTER TABLE varer ADD CONSTRAINT varenavn FOREIGN KEY (varenavn)
REFERENCES languages(relid) MATCH FULL ON DELETE CASCADE;

 >ERROR:  there is no unique constraint matching given keys for
referenced table "languages"
tostipippitest=#

Can't i create a Foreign key to a field that is not defined as UNIQUE?

And if so, are there any other approach to solve this problem?

Sincerely

Victor

PS If someone has a link to a good tutorial I would love to have it DS