The European Parliament's chief of staff has urged MEPs to speak slower and stick to their native language, to help the interpreters.
Secretary General Klaus Welle said: "It's extremely important that people do not speak too fast." Interpreters had made that request, he added.
Thousands of interpreters and translators work in EU institutions, to cope with 24 official languages.
The parliament has about 330 staff interpreters and 1,800 freelancers.
In addition, the parliament employs about 700 translators, who translate more than 100,000 pages each month.
The most recent languages to be made official were Croatian (in 2013), Irish, Bulgarian and Romanian (all in 2007).
Interpreters translate orally, in real time. But Mr Welle said some were overburdened and others under-employed.
"The average amount of time in the interpreter's booth is 10 hours... But we need more fairness between interpreters - the individual tally varies between six and 16 hours in the booth," Mr Welle said.
The scope of translation work in the EU was unique, he said. "Even at the UN you only have interpretation in six languages."
"It's also extremely important for interpreters that people speak their own language," he told the parliament's budgetary control committee.
"If they speak a foreign language the quality of interpretation goes down. You hear interpreters making requests to you: 'Please speak more slowly, speak your own language.'"
The parliament's budget for interpreters is €45m and another €9m for translations done externally.
The European Commission - where EU laws are drafted - has 600 staff interpreters and 3,000 freelance interpreters. The Commission's total staff is about 33,000.