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.
Plagiarism is presenting other peoples' work as your own. Since the described situation is not mentioning work by other people, it cannot be plagiarism.
Now "This is not plagiarism" is a necessary, but not a sufficient criterion for being acceptable.
It could be self-plagiarism (which is very unfortunately named, since it is not a kind of plagiarism). Self-plagiarism is a bit ill-defined, it is essentially about attempting to be credited twice for one bit of work. Since a PhD thesis is fundamentally different from a non-refereed article, self-plagiarism does not apply.
It could be prohibited by the relevant regulations of your university. Check those to see if this applies.
Including the translated material without being transparent about it would violate good scientific practise.
Finally, there could be copyright issues, if you are not the copyright holder for the translated article, and don't have the right reserved to use for your thesis. It is up to you whether you care about copyright, in practise you can typically get away without doing so.
tl;dr: Not plagiarism but try to avoid quoting long passages
If you explain what you're doing, i.e. either cite your own non-English publication or mention clearly that some parts of the work are translated from a non-English publication (say, as part of an introduction or an overview) - then you've certainly not committed any sort of plagiarism nor anything wrong ethically or morally. Still, like @WanderingChemist suggests, this may be against technical regulations.
Having said that: It's rarely a good idea to quote large sections of text verbatim. So unless there's a very good reason not to, I suggest you do some rephrasing/restructuring to fit with your thesis:
- Use unified preliminaries/definitions/terminology
- Refer to findings in other parts of your thesis which were not referred to in the non-English paper
- Consider refining, elaborating or illustrating your argumentation; this is a thesis, after all, so you're not under stringent restriction of the number of pages.