Primary codes in Laravel

For this new problem that you pose this is my answer.
In the documentation you have all the different types of indexes and how you can generate them. You are in need of a single composite index and that is like the composite index that I put in the previous answer only that instead of being a primary index in this case it is a unique index. In this way you will be able to have as many languages for each user as you wish.

Schema::create('idiomas', function (Blueprint $table) {
    $table->increments('id');
    $table->string('idioma');
    $table->unsignedInteger('user_id');
    $table->foreign('user_id')->references('id')->on('users');

    $table->unique(['user_id', 'idioma']);
});

I recommend you read the documentation Creating Indexes to understand a little better.
Greetings and I hope I have been helpful

You should have a table users , a table idiomas and another table idioma_user . The latter to describe the languages available to users.

I propose the following:

Migrations

Schema::create('users', function (Blueprint $table) {
    $table->increments('id');
    $table->string('name');
    $table->string('email')->unique();
    $table->string('password');
    $table->rememberToken();
    $table->timestamps();
});

Schema::create('idiomas', function (Blueprint $table) {
    $table->increments('id');
    $table->string('nombre');
    $table->timestamps();
});

Schema::create('idioma_user', function (Blueprint $table) { // tiene que ir en orden alfabetico y en singular al igual que los nombres de los modelos correspondientes
    $table->increments('id');
    $table->unsignedInteger('user_id');
    $table->foreign('user_id')->references('id')->on('users');
    $table->unsignedInteger('idioma_id');
    $table->foreign('idioma_id')->references('id')->on('idiomas');
    $table->string('nivel_hablado');
    $table->string('nivel_escrito');
    $table->string('titulo_oficial');
    $table->timestamps();

    $table->index(['user_id', 'idioma_id']); // con esto no podrá haber un usuario con 2 idiomas. solo puede existir esta combinación.
});

User Model

namespace App;

use Illuminate\Notifications\Notifiable;
use Illuminate\Foundation\Auth\User as Authenticatable;

class User extends Authenticatable
{
    use Notifiable;

    protected $fillable = [
        'name', 'email', 'password',
    ];

    protected $hidden = [
        'password', 'remember_token',
    ];

    public function idioma()
    {
        return $this->hasOne(\App\IdiomaUser::class);
    }
}

Model Language

namespace App;

use Illuminate\Database\Eloquent\Model;

class Idioma extends Model
{
    protected $fillable = [
        'name',
    ];

    public function users()
    {
        return $this->hasManyThrough(\App\IdiomaUser::class);
    }
}

IdiomaUser Model

namespace App;

use Illuminate\Database\Eloquent\Model;

class IdiomaUser extends Model
{
    protected $fillable = [
        'user_id', 'idioma_id', 'nivel_hablado', 'nivel_escrito', 'titulo_oficial'
    ];

    public function user()
    {
        return $this->belongsTo(\App\User::class, 'user_id');
    }

    public function idioma()
    {
        return $this->belongsTo(\App\Idioma::class, 'user_id');
    }
}

With this you can for example do:

$user = auth()->user();
dd($user->idioma, $user->idioma->nivel_hablado);

You could also get the list of users by language

$idioma = Idioma::find(1);
dd($idioma->users);