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