An ETSOA can be validated in a third country provided there is a bilateral agreement or working arrangement in place; see the list published here. Applications should be made through EASA Form 41. Based on the bilateral agreement, EASA will carry out the technical investigation on behalf of the foreign authority. EASA will provide you with a quote on how many hours would be needed to perform this technical investigation.
Please note that some foreign authorities have special policies regarding validations. The applicable technical documents should be provided in electronic format (no paper), also for FAA validations. Please submit your technical data to the ETSOA mail box (etsoa [at] easa.europa.eu) and copy the responsible PCM. Files larger than 10 MB should be submitted via EASA filebox, https://filebox.easa.europa.eu/
(Please copy the PCM and etsoa [at] easa.europa.eu).