Hawk on Kremlin and Ukraine crisis signals he will take job as European council president if there is a consensus behind him

The Polish prime minister, Donald Tusk, is expected to become the co-leader of the European Union when EU government chiefs meet to decide who will head the union for the next five years.

Tusk, a leading EU hawk on the Kremlin and the crisis in Ukraine, is set to become the second president of the European council, the top official who chairs EU summits and mediates between the 28 governments. He has come under strong pressure over the past week to accept the post andsignalled on Friday that he would take the job if there was a consensus behind him.

Tusk's difficulty is that he speaks no French and only poor English, the lingua franca of the summits and also essential in representing the EU abroad and to the media. He understands German. Gazeta Wyborcza, the leading Polish daily newspaper, reported that Tusk's wife was keen on the move to Brussels as it would mean more money, more prestige, and more time off.

The 28 leaders are to meet in Brussels on Saturday evening to decide the top job and appoint a new EU foreign policy chief to replace Britain's Catherine Ashton, who was the first holder of the post.

She is almost certain to be replaced by Federica Mogherini, the Italian foreign minister who has been pushed strongly by Matteo Renzi, the Italian prime minister, and backed by the EU's social democratic leaders.

Federica Mogherini, the Italian foreign minister, is almost certain to replace Britain's Catherine Ashton as the EU foreign policy chief. Photograph: GEORGES GOBET/AFP

Mogherini was also opposed by the Poles and other east Europeans last month as she was seen as too soft on Russia and its president, Vladimir Putin. The emergence of the Tusk-Mogherini ticket removes that opposition as Poland has been leading the campaign for a more energetic anti-Putin and pro-Ukraine policy.

Senior EU officials involved in the negotiations said the summit was certain to make the two appointments although names were not confirmed.

The Danish prime minister, Helle Thorning-Schmidt, a social democrat, has also been a frontrunner to replace Herman Van Rompuy of Belgium, the incumbent president who will chair the summit and propose the names.

The Mogherini nomination, EU officials admitted, confirmed the relative lack of clout going with the job at a time of barely precedented foreign policy challenges around the EU's borders.

The 41-year-old has been Italian foreign minister for six months. Her appointment has been accompanied by a complete absence of discussion of European foreign policy, aims, strategies, and resources.

The outgoing Ashton was also handicapped by the reluctance of national capitals and foreign ministries to surrender powers over foreign policy. She is widely seen to have underperformed, while notching up notable achievements in the Balkans and in mediating nuclear crisis talks with Iran.

The two posts have to be decided on Saturday in order to enable the new head of the European commission, Jean-Claude Juncker, to get on with forming his team of 28 commissioners over the next fortnight.

Their appointment unlocks the complicated set of criteria determining commission portfolios – involving party political alignment, gender and nationality.

Juncker, the former Luxemburg PM, replaces José Manuel Barroso on 1 November. With the exception of foreign policy, Juncker has power over the allocation of portfolios. The economic jobs are the most coveted, but he has refused so far to show his hand.

Juncker's main problem is the lack of women. Barroso's outgoing commission has nine women and so far governments have sent Juncker only four, including Mogherini. Gianni Pittella, the leader of the socialists in the European parliament said his group would oppose "a college of European commissioners with fewer women than today".

While the German chancellor, Angela Merkel, has been Tusk's strongest backer, David Cameron came out this week in his support despite previous frictions over the British prime minister's attempts to curb freedom of movement in the EU and calls to clamp down on welfare benefits for EU migrants, both policies hitting the large Polish community in the UK.

While the summit was called to focus on the leadership appointments after last month's failure to agree on anything, it looks likely to be hijacked by the Ukraine emergency and the Russian invasion in the east of the country.

Putin's boldness and his timing is particularly an affront to Merkel who is leading the diplomatic effort in Europe and who was in Ukraine only last weekend.

• This article was amended on 30 August 2014. The original version wrongly referred to Gianni Pittella as "her". This has been corrected.