nb-select not work with ngx-translate

### Issue type

**I'm submitting a ...** (check one with "x")

* [x] bug report
* [ ] feature request

### Issue description

**Current behavior:**
When language change, the selected text does not change

**Expected behavior:**
When language change, the selected text should change

**Steps to reproduce:**

**Related code:**

```

{{ 'enabled' | translate }}
{{ 'disabled' | translate }}

```

### Other information:

**npm, node, OS, Browser**
```
node: 10.15.3
npm: 6.4.1
OS: Linux
Browser: Chrome
```

**Angular, Nebular**
```
Angular: 8.0.0
Nebular: 4.1.2
```