I have installed several plugins, but there are a lot of, which doesn't support my language. I found solutions, which supports to write multilingual content, but can't handle language of plugins or maybe translate it automatically with help of some webservice.
Poedit is the way to go fro translating most of your plugins.
WPMl is doing quite a good job, though, at parsing plugins and themes strings for translation purposes. You should give it a try.
There are many translation plugins. I'd say check out
Loco Translate: https://en-ca.wordpress.org/plugins/loco-translate/
- Built-in translation editor within WordPress admin
- Create and update language files directly in your theme or plugin
- Extraction of translatable strings from your source code
- Native MO file compilation without the need for Gettext on your
- Support for PO features including comments, references and plural forms
- PO source view with clickable source code references
- Protected language directory for saving custom translations
- Configurable PO file backups
- Built-in WordPress locale codes