As per ICAO Annex 10PDF, it is no longer a recommendation, but a requirement that all controllers be able to speak English with an ICAO language proficiency of at least level 4. ICAO Annex 10 states:
22.214.171.124 Language to be used
126.96.36.199.1 The air-ground radiotelephony communications
shall be conducted in the language normally used by the
station on the ground or in the English language.
Note 1.— The language normally used by the station on the
ground may not necessarily be the language of the State in
which it is located. A common language may be agreed upon
regionally as a requirement for stations on the ground in that
Note 2.— The level of language proficiency required for
aeronautical radiotelephony communications is specified in
the Appendix to Annex 1.
188.8.131.52.2 The English language shall be available, on
request from any aircraft station, at all stations on the ground
serving designated airports and routes used by international air
184.108.40.206.3 The languages available at a given station on the
ground shall form part of the Aeronautical Information
Publications and other published aeronautical information
concerning such facilities.
This however doesn't mean that countries cannot still work with their own language and countries like France still work IFR & VFR traffic in their local langauge, whereas some countries like Germany limit German to VFR traffic and emergencies, where plain language can be used. IFR traffic in Germany is conducted in English only.