In general I put in name:en what I find on signs or plates, except there’s a typo. I take it as official international name and because Thailand is using english words in their international names I put it in name:en. Sometimes it’s a translation (e.g. Road), sometimes a romanization (e.g. Thanon), sometimes it’s a mixture (e.g. Kasikorn Bank). If there are different versions on different signs I take which I think suits best. Sometimes I put two versions in name:en. This is to help a foreigner who is using the map and reading the signs when looking for a road or location.
To talk to lokals who are not able to read the international name in roman letters on the signs and don’t know it I think the best is to use bilingual maps. The local can read the Thai name. And the foreigner even with little skill of Thai can try to compare letter by letter if there’s no international name on the sign. And that’s why I add the local name and the international name whenever I can.