here's the info in the wiki:
"Language facilities

Next to the completely bilingual Brussels, there are also municipalities with language facilities (see the map).

These municipalities are supposed to operate in the official language of the community, but must provide services to minority language groups upon request. This means f.e. that all streetnames also have an official translation and are normally signed with the translation too. But there's no language-equality as in Brussels.

These municipalities should have the official translated names under name:nl=*, name:fr=* and name:de=* (depending on which language facilities are offered), the name=* tag should contain only the official name in the primary language. For the moment, the municipalities bordering Brussels also follow the naming schema used in Brussels (with both languages in the name tag), while the other municipalities have a tendency to only include a single language in the name tag. Do not delete translated data by following this rule. If a good translation exists in a dutch municipality in french, do not remove it. We only care for correct data, as much as possible without being intrusive in any way. "
https://wiki.openstreetmap.org/wiki/Zh-hans:Multilingual_names#Language_facilities