In addition to @Mr.TaxOpinion's answer:
I've been in a similar situation (Germany/Italy, though), and I found speaking to the tax office very helpful. They cannot and will not give you tax advise, but they have to and will explain to you:
- what conditions apply to you in order to "use" the double taxation treaty
- what documentation you need to produce, and
- where to fill in what into their forms.
(And don't forget, while a tax advisor gives advise, you anyways stay responsible for your tax declaration)
For my German/Italian working experience:
- The German tax office first asked me to fill in a form that they use to find out whether you are subject to unlimited/unlimited income taxation.
- Double taxation treaty -> foreign income is not taxed again in Germany.
I highly recommend that you read the one relevant for you: They are all listed at the federal ministry of finances web site.
- This is subject to the condition that I needed to attach proof that the foreign income was properly treated at the foreign county. (In my case, the Italian income was subject to 0 % income tax, and I submitted pay slips stating this together with the text of the law and a [non-official] translation into German.)
- However, the foreign income did enter the calculation of total income (Progressionsvorbehalt), so it had to be listed in the income tax declaration.
My experience here is also that this is likely sufficiently compley that the tax officer at the service line will refer you to someone else (possibly the tax office to whom you "belong"), and they in turn will tell you that they'll have to look up details themselves ("Could you kindly call again mid of next week"), but that communication "mode" worked well.
When consulting a tax advisor on such questions, I'd ask for an approximate quote beforehand. Also, you can ask the chamber of tax advisors who is specialized on such questions of German/Thai income tax (there are also search engines on the web).