I would like to refer you to a Supreme Court of Canada case. I'm talking about Société des Acadiens et Acadiennes du Nouveau-Brunswick v. Canada, from 2008.
The issue you raised is twofold. The RCMP is a special organization. The Supreme Court has established that the RCMP is a federal institution at all times. So, as a member of a federal institution at all times, when an RCMP officer arrests an individual, they must comply with section 20 of the charter. Under that section, in terms of communications and services, the officer must provide an active offer. That is covered in part 4 of the Official Languages Act, which also stipulates that the police officer must provide an active offer. So the officer must speak to the arrested individual in French. They could say the following:
"Bonjour madame/monsieur. Would you like to be spoken to in English or in French?"
Now here is the problem. The issue of significant demand under section 20 of the charter comes into play. There are also official languages regulations that outline specific territories, events and contexts where services should be offered in both languages. Everyone is entitled to a hearing in their language, so we are no longer talking about significant demands.
This has to do with the concept underlying section 19 of the charter. When the evidence before a judge violates certain rights, the judge cannot take it into consideration. If the officer violated section 20 of the charter, the situation becomes problematic.