@Ali *****
In my experience the answer is “Yes the Thai Embassy in London need to verify a translation of the marriage certificate”
In my experience the process is as follows:
1. IN THE UK: Marriage certificate legalised by the FCDO in Milton Keynes
2. IN THE UK Marriage Certificate AND the Apostille from the FCDO Translated
3. IN THE UK: Marriage Certificate AND the Apostille from the FCDO AND their translations verified by the Thai Embassy in London
4. IN THAILAND: All the above (ie all 4 documents & their translations, making 8 in total) Legalised by MFA in Bangkok
The above is the process I successfully followed in 2023 to get the Kor Ror 22 which was used for the Non-O Visa & the 1 year Extension based on Marriage.
The RTE London has a list of “approved” translators on their website. Mine also did the verification of all the documents with RTE London for me.