Is translating my own work to use it in my PhD thesis plagiarism? Strictly speaking, is translating to english my own published work written in my mothertongue in a non-peer reviewed journal to put it as a part of my PhD thesis plagiarism ?
This will depend on your institution's rules and your situation, but here are some general points


You can usually use any (and only) work you have performed as part of your PhD studies* to satisfy your thesis requirements (previous work you've done, other than as part of your PhD, is typically included as if you were referring to the literature). Non-standard programmes, such as professional doctorates and DSc qualifications, may have different rules on this.
You will need to be mindful of whether you're allowed to copy word-for-word. In the UK, for instance, you can use results from your publications in your thesis, but you have to restructure the text.** In other countries, thesis-by-publication is more common, and a direct translation may be permissible.
I strongly recommend appropriate references to/mentions of your existing publications - this could take the form of a page at the front entitled "publications arising from this work".
You will need to be aware that direct reuse could create copyright issues, if you intend to put your thesis online or republish it. You will need to investigate the copyright status of your publications if you intend to do this. This answer explains how to acquire rights, if needed. This answer covers some copyright considerations in more detail.


*The meaning of this is a bit of a grey area. But this usually means performed during your time of registration, towards your registered project, at or under the auspices of the institution(s) you are registered with/official project sponsors.

**This is a consequence of both academic regulations often precluding thesis by publication and the different requirements of a thesis and a few independent papers - in particular, the requirement that a thesis forms a coherent whole, and a typical need for more context and detail in a thesis, vs conciseness in a paper.